These aren't comparable examples. If you hire a hitman, there's a reasonable expectation that people will die. No such expectation exists for hiring someone to mow your lawn.
Under what circumstances is one responsible for someone else's acts. Just because you hire someone to do something illegal, you are not responsible for all the illegal acts the other person might commit.
The mere fact of putting something in motion does not immediately mean one is responsible for all the outcomes of all actions in the chain.
There's a reasonable expectation that the victim will die, but not anyone else. I guess you can argue that there might be a risk of collateral damage, but there's also a risk of collateral damage in hiring contractors for your house (eg. the plumber starting a gas leak resulting you and your neighbor's house getting blown up).
