Assassinations and Targeted Killings in Halacha – Donald Trump, Mohammad Deif, etc.

What’s the rationale to halachically permit targeted killings of Israel’s enemies?
Is there any difference between a targeted killing during an active war and when there’s no active war being fought?
What if there’s only a 10%, 20% or 30% chance that the target is where you think he is?
Do you need to be concerned about collateral damage during a targeted killing? And, what if it could be significant?
Is there a problem celebrating when our enemies are killed?