Thou shalt not kill, that's all there is to it really
If you provide tools that help an organization kill easier, and possibly lazily rely on your inaccurate tools' judgment, and you do not care about the outcomes of these tools' uses, that's on your soul, too.
Yes, causality and attribution are hard sometimes, but it may have been that without it, hundreds of humans would be alive right now, and he doesn't even know. Maybe he should talk to the kids' parents about what alignment means.
Ah, I didn't realize we're having a religous discussion. If all there is to it is "thou shalt not kill" then I guess that according to your POV he shouldn't have sold to the military even if the system was only suggesting "valid" targets.