Bennett shows that he is curious and passionate about knowledge when it comes to things he feels emotionally invested in. His hobby is something he excels at and he's portrayed as studious and possibly also a fast learner.
He seems to be striving for personal growth, not just strength, and that's what makes me believe that he is in fact redeemable, because while he can't undo the mistakes he's made, he does have the potential to see the error of his ways and improve to be a better person.
I forgot who brought up the possibility of Bennett's character theme being metamorphosis, but I like that idea a lot and see it as a likely outcome.
Bennett is someone who needs to learn to guide his determination and passion with more care, instead of acting on greed.
That's really what he needs to understand the most - that he is being greedy about what he wants, and that he doesn't have a right to everything he desires.
I dislike the idea that a character is "irredeemable" after they've done a bad thing. Bennett still has plenty of chances to prove himself as a better person than he is now.
His immaturity and lacking understanding of boundaries is nowhere near the level of characters like Sirius (who legitimately feels neither sympathy nor pity for anyone else). He's just that - an immature boy who is being emotionally manipulated by a far older person with ulterior motives. He is being exploited, and this is what turned him into the kind of character we currently know him as. El knew exactly what he was doing.
Bennett may not be a super sympathetic character who we'd all rush to protect with our lives, but with all his flaws, he didn't turn into who he is right now completely on his own accord. He just needs a chance to redeem himself, and I think he'll eventually get it.