Darwin was clearly just there to die. His powers are lame, he raises the stakes... I'm fine with it. (I have no prior knowledge or attachment to the character - and he had a fairly heroic and cool death scene!)
His death scene bothered me because it contradicted his power.
"I adapt to survive."
Well, apparently not.
It's the age old "This is how powerful the villain is, by showing him take down the biggest and strongest".
In this case, Darwin is the toughest to kill, therefore the most dangerous. By his very nature he can't die. How do you absolutely show that Shaw is not to be messed with? Have him (not his henchmen) kill the unkillable.
And then get that to motivate the youngins, who, up until that point, weren't taking things as serious as they should.