One stock objection I've seen pacifists use is to equate killing somebody with "hating your enemy." There are several problems with that argument:
i) I may not be killing my enemy. Rather, I may be killing your enemy, to protect you from wrongful aggression.
ii) In addition, why assume that a soldier (to take a stock example) must "hate" the enemy? Whether or not hatred is involved depends on the soldier and depends on the enemy.
Some enemies are hateful. Some soldiers hate the enemy. But that's hardly something we can universalize.
i) A soldier may kill the enemy simply because he's been ordered into combat. He goes wherever he's told to go. He may personally dislike killing. He's just following orders.
ii) He may regard killing as a necessary evil. He'd prefer not to kill anyone, but losing is worse.
iii) He may believe in the mission, but that doesn't necessarily mean he entertains personal animus towards enemy combatants.
iv) He may kill, not because he hates the enemy, but because he finds himself in a kill-or-be-killed situation. Given a choice, he'd rather survive.
v) He may kill to protect his comrades. Take a military sniper. It's not so much a question of what he feels about the enemy, but what he feels about his comrades.
vi) He may kill because he has to make a split-second decision. It's not premeditated. He doesn't have time to feel anything about the enemy. The only thing he feels is adrenalin.
vii) He may be pretty detached. Take dropping bombs at high altitude. Or launching a cruise missile. He never sees the enemy. It's fairly abstract. He doesn't feel much of anything, one way or the other. He doesn't have a mental image of the enemy. It's about the target. The strategic objective.
viii) He may be sympathetic to the plight of enemy conscripts who are there because they were drafted, and not because they want to fight.
In many or most cases, a soldier may regret having to kill the enemy. He must overcome his innate reluctance to be responsible for someone else's death, even if it's justified in his eyes.
i) I think pacifists caricature soldiers. That serves their polemical purpose to oversimplify the psychology of a soldier.
ii) In addition, the moralistic affectation of the pacifist prevents him from assuming a soldier's viewpoint, even for the sake of argument. Because he thinks killing is evil, he thinks it's evil to think like a soldier, even for purposes of understanding the other side. So it's more convenient to impute malign motives to the soldier.