How do any police who corner what they think is a murderer know what they think is right? All they can do is pursue those they think responsible, ask them to give themselves up and if there is a refusal to give up, then use force. Where the suspicion is that the culprits are murderers, lethal force is warranted and used throughout the world. What's missing in your analysis is that these Pal thugs did not give the settler any opportunity to defend himself for his terrible sin, according to these thugs, of trespass.
Second rocket fired from Gaza explodes inside Strip (Haaretz)