Why not. Our guys do it all the time - even in this country, even U.S. citizens.
Because it is wrong? This is simple stuff. Countries cannot go kill people including innocent bystanders with chemicals whenever they feel like it is right. There are legitimate self-defense reasons to conduct drone strikes although those should be limited to situations where there is an imminent threat and it is too risky to do anything else. If Putin wants to conduct a drone strike on some ISIS target I would support that. I do not support Putin killing a spy and his daughter possibly along with other bystanders or police.