Why kill useful workers?
Humans are herd animals, it is relatively easy to keep them inline. Especially if most are unaware that they are being kept inline.
The easiest approach would be to gain trade access, followed up by enlarging their own population while simultaneously causing attrition to the indigenous population. The last step would be complete integration or annihilation.
To that end, they would land somewhere relatively innocuous, but with a handful of pliant, charismatic, and socially acceptable individuals. A quick analysis of the planets primitive communication network should reveal any number of such individuals.
These individuals are provided with information and designs well in advance of anything available in this primitive backwater, but still quite old tech, or perhaps some insight into the market that could be passed for believable intuition. This gives a gateway for aliens to trade with the local population (via these individuals/companies) gaining access to the planetary workforce, and production.
Having access to the market provides the means to replenish, repair, and rebuild what the refugees had lost. This gives the simplest access to processed goods such as food, water, fabrics, and raw resources. If the right businesses were established this can also give access to the industrial machines responsible for refining, shaping, and assembling.
Being able to source even relatively primitive manufacturing capacities can serve to relieve load off the limited manufacturing capacity the refugees still have. Even primitive materials can have useful applications. If the refugees lost their own ability to manufacture, this is even more important, as it will allow them to cobble together manufacturing capabilities in order to repair their ships, or expand with other necessary infrastructure projects that cannot be built by the indigenous humans.
After sometime there exists the possibility that the selected individuals become too risky - accidents can be arranged. By this point enough local customs have been learnt to train several special operatives. Through some form of remote control, reconstructive surgery, or holographic imagery they can pass as human. So replacing the individual with a "foreign business man" isn't too unreasonable.
Over the next few years the company goes from strength to strength. It is now generating revenues comparable to several small countries and large states. Time to get into politics. Bribery, thought policing, back room deals, advertising, and even outright campaigning to install suitable leaders. Perhaps even start wars between several nations which are already in conflict. Perhaps even influence the culture to support destructive cultural norms to reduce their overall birth to death rate.
By the time the humans are even aware of the alien presence it is already too late. Either most of population are alien/human hybrids, or the aliens significantly out number the humans, or there is no way for the humans to out compete the aliens and while they may not be defeated, it is only a matter of time.