When Will says it’s beautiful, he’s not saying that the act of killing is beautiful, rather he finds reclaiming his power beautiful. A motivator to Will in the story is the constant robbery of his power and agency. You see s1, Hannibal meticulously dismantles his world and traps him helplessly. What’s great about Will and is characteristic to him is that he’s attracted to power, and interestingly, when it’s ripped away from him, he turns violent and cunning. What Dolarhyde does to him—firstly the attack on Will’s ideals by attacking families (his sore spot), indulging in a perverse mentorship with Hannibal (another sore spot; not in the jealousy context, but anything in his life with Hannibal in it is a perpetually healing scab), and attempting to murder Molly and Wally (in his perception, innocents). First, he cannot figure out Dolarhyde’s pattern (What about these families make him target them?), Hannibal refuses to help him (Why must he play mind games with an immovable, selfish creature who shakes everything that he is), and he cannot do anything to save Molly and Wally until he gets the call they’re in the hospital. Add to that, dangling Frederick as a bait (desperation making him dangerously violent, again), which spits in his face. In the end the only thing he can do is give in to entropy (release the very volatile Hannibal into the environment) and reclaim as much as he can. He ends up reclaiming his power from one, killing Dolarhyde, and two, killing Hannibal (and himself), and that is such a high and it’s beautiful what he is capable of.