Part One: Ethics.
I note that Europa has a water ocean beneath its thick surface layer of ice. For decades there has been much speculation about whether there could be some liquid water using lifeforms in the subsurface ocean of Europea.
So that give a few possiblities for your story:
Investigation of the subsurface ocean and/or theoretical studies have shown that there is no life in the subsurface ocean of Europea, and thus it is ethically proper to terraform Europa.
Investigation of the subsurface ocean and/or theoretical studies have shown that there is no life in the subsurface ocean of Europea, and thus it is ethically proper to terraform Europa. Later, during the terraforming or afterwards, it is discovered that there was an error and there is now alien life in the oceans of Europea, or that the terraforming killed all that life.
No body bothers to study the possibility of life in the subsurface ocean of Europa before beginning the terraforming. Thus it might be daccidentialy discovered that such life exists or did exist.
It was proved that life existed in the subsurface ocean of Europa, but the decision was made to terraform Europa anyway.
Part Two: Retaining an Atmospherev; Magnetosphere.
If Europa is terraformed to have a dense atmosphere, it will have trouble retaining that dense atmosphere.
Europa has no magnetosphere to deflect particles in the solar wind, which might knock off atmosphere particles which would be lost into space.
At Jupiter's distance from the Sun, the density of solar wind particles should be only about one percent of the density at Earth's distance from the Sun, which might greatly reduce the rate at which atmospheric particles are lost.
Europa might orbit within the magnetosphere of Jupiter, which might protect Europa from solar wind particles and prevent them from eroding the atmosphere of Europa.
In "Magnetic shielding of exomoons beyond the planetary habitable edge", Rene Heller and Jorge Zuluaga seem to calculated that a moon of a giant planet should orbit at a distance of between about 5 and 20 planetary radii to be within the magnetosphere.
https://arxiv.org/pdf/1309.0811.pdf
Jupiter has a radius of 69,911 kilometers, so 5 to 20 Jupiter radii would be about 349,555 to 1,398,220 kilometers. Europa orbits Jupiter at a distance of about 670,900 kilometers, so it should have protection from Jupiter's magnetosphere.
But if the less dense solar wind and the magnetosphere of Jupiter don't protect Europa's atmosphere enough the terraforming project will have to create some sort of physical or magnetic shield to protect Europa's atmosphere from the solar wind.
Part Three: Retaining an Atmosphere; Escape Velocity.
But a much more important factor to consider is the mass and gravity of Europa. Europa, being largely made of water, has a low overall density, which combines with its small size to give it a low mass. Europa has a mean density of 3.013 grams per cubic centimeter, 0.546 that of Earth. Europa has a radius of 1560.8 kilometers, 0.245 that of Earth, which gives it a volume of 1.593 times ten to the tenth power cubic kilometers, 0.015 that of Earth. Thus Europa has a mass of 4.799844 times 10 to the 22nd power kilograms, 0.008 that of Earth.
Note that none of those properties of Europa has the same ratio to Earth's as any other property does.
The surface gravity of Europa, important for human comfort on Europa, is 1.314 meters per second per second, which is 0.1339 that of Earth's 9.80665 meters per second per second.
The escape velocity of Europa, vital for retaining an atmosphere, is 2.025 kilometers per second, 0.1810298 of Earth's escape velocity of 11.186 kilometers per second. As you see it has a different ratio to Earth's than the surface gravity does.
I don't know if you want terraformed Europa to be habitable for liquid water using life in general, or for humans in particular. Worlds habitable for humans are a smaller subset of the set of worlds which are habitable for liquid water using life in general.
The main discussion of habitablity for humans in particular that I know of is Habitable Planets for Man, Stephen H. Dole, 1964. https://www.rand.org/content/dam/rand/pubs/commercial_books/2007/RAND_CB179-1.pdf
The hotter a gas is, the faster the atoms and molecules in that gas will be moving. The heavier a gas atom or molecule is, the slower it will move at a specfic speed. In a gas at a specific temperature, some elements and compounds will be moving faster than others.
According to pages 34 and 35, there is a rough mathematical correlation between the length of time that it take for a world to loose enough of a gas to reduce the amount to 1/e or 0.368 of the original amount and the ratio of the escape velocity of that world divided by the the root-mean-square velocity of particles of a specific gas at a specific stemperature.
According to table 5 on page 35, the length of time for a gas in a world's atmosphere to be reduced to 1/e, 0.368, of its orginal amount would be approximately zero time if the ratio of the escape velocity divided by the root-mean-square velocity of that gas in the exosphere of the atmosphere is 1 or 2.
If the ratio is 3, the world will loose so much of a gas it will have only 0.368 of it left in the atmosphere after a few weeks. If the ratio is 4, the world will loose so much of a gas it will have only 0.368 of it left in the atmosphere after several thousand years. If the ratio is 5, the world will loose so much of a gas it will have only 0.368 of it left in the atmosphere after about 100 million years. If the ratio is 6 or higher, the world will loose so much of a gas it will have only 0.368 of it left in the atmosphere after an approximately infinite time.
The escape velocity of Europa is 2.025 kilometers per second.
So if any gases in the atmosphere of terraformed Europa have a root-mean-square velocity of 1.0125 or higher in the exosphere of Europa, they will escape so fast that their amounts will drop to 0.368 of their original amounts instantly.
If any gases in the atmosphere of terraformed Europa have a root-mean-square velocity of 0.75 kilometers per second in the exosphere of Europa, they will escape so fast that their amounts will drop to 0.368 of their original amounts in a few weeks.
If any gases in the atmosphere of terraformed Europa have a root-mean-square velocity of 0.50625 kilometers per second in the exosphere of Europa, they will escape so fast that their amounts will drop to 0.368 of their original amounts in several thousand years.
If any gases in the atmosphere of terraformed Europa have a root-mean-square velocity of 0.405 kilometers per second in the exosphere of Europa, they will escape at a rate that their amounts will drop to 0.368 of their original amounts in about a hundred million years.
If any gases in the atmosphere of terraformed Europa have a root-mean-square velocity of 0.3375 kilometers per second or lower in the exosphere of Europa, they will escape at a rate that their amounts will take approximately an infinite time to drop to 0.368 of their original amounts.
And i suspect that molecules and atoms of nitrogen, oxygen, carbon dioxide, water vapor, and other gases you want to have in the atmosphere of terraformed Europa will have much higher velocities in the exosphere of Europa than 0.6675, 0.405, or 0.50625, kilometers per second.
Thus you need to find a way to help Europa kept its atmosphere for long enough for it to be worthwhle terraforming Europa.
And fortuantely my answer dated Tues. january 25, 2022 at this question: How to make an Old Solar System scientifically possible
And also in my answer on 01-22-22 to: How long can this planet hold its atmosphere?
