If you're talking about a creature that is already smart enough to use tools and work together to hunt and gather, the most likely reason that they would become so large is an abundance of resources and constant competition with other members of the species. The use of weapons, traps and cooperation can overcome larger or tougher predators or prey without the constant resource cost of feeding large muscles (we are as we are because working smart pays better dividends than working hard). We have evidence of prehistoric humans hunting mammoths and woolly rhinos, and there doesn't seem to have been much selective pressure for humans to increase in size. However, if these hominids were constantly forced to defend their territory against other, equally intelligent hominids, you might see an evolutionary arms race to produce the toughest warriors that the environment would bear. Of course, selection pressures could just as easily go the other way and produce smaller, more food-efficient hominids that can beat their larger brethren through numbers rather than brute force.
As far as environment, it would probably require an environment with high biological productivity, so most likely a tropical forest or wetland, or possibly a coastal region with plentiful fishing. If resources were more scarce, or more variable there would likely have been a strong selective pressure to stay small, as the larger an organism is, the more food it needs to sustain itself. One way to get around this is for some feature of the local environment, flora or fauna to actively aggregate resources for you. As an example, if water were scarce and your hominids were to prey upon those trying to drink from rivers lakes or watering holes, the movement of their prey would do a great deal of their work for them.
If you're going big, you're also going to wind up with a species that tends to reproduce more slowly, and develop more slowly than smaller cousins, so they aren't going to be able to sustain a high attrition rate. Combat will either need to be restricted to males (in this case the disparity in mortality rates will favor polygamy) as is seen in lions or gorillas or combat will have to be mostly "ritualized" with very few fights actually being to the death, as you see in most big cats. Remember, for every fight to the death that you expect an individual to experience in their life, you have just halved whatever portion of the population is engaged in fighting.
The limiting factor with a large-bodied hominid, as with any large animal, is population density. Any given environment only receives so much sunlight and water, and will only produce edible biomass at a finite rate. As your organism gets larger, it needs more and more territory to sustain itself. If these organisms gather together in family or community groups, not only does this increase the size of the territory they need in proportion to their number, but it also increases the distance they must travel each day in search of food, especially if they all travel together. Ultimately groups will reach a size where there are simply not enough hours in a day to gather food for them all, and the group will have to split up.
These larger stronger hominids will require not just more calories per person, but possibly more calories per unit mass, meaning that though they are individually more fearsome, they would live in smaller groups. Gorillas, which are very similar in mass to your proposed hominids live in groups of about 10-12 individuals. By contrast it's estimated that prehistoric human groups consisted of typically around 25-50 individuals. One possible consequence of this is that they may be less socially intelligent than humans, as their group dynamics will tend to be simpler all else being equal. Of course, they could all be machiavellian schemers waiting for the perfect moment to stab each other in the back, so that even though the social network has fewer connections, each connection is more complicated, and requires more attention.