I'll spoiler this in case anybody else has bought it:
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The whole thing is a revenge thriller, dying to get revenge is in keeping with the sort of movie they're aping.
For me, I mean. I totally get why it'd piss you off and to be honest I'm not sure why it doesn't piss me off because I'm normally the first to rage at games for changing the rules as it suits them or for a final boss.
I hope I'm not turning into some "it's an artistic decision, maaaaaaan....!" dweeb.
I'm going to give it a go over the weekend but it occurs to me that given I'm not ever going to kill every enemy on that level I'm probably trying to kill way more than I actually need to.