Can't help but wonder if this isn't perhaps an evolutionary trait.
60 year old men aren't supposed to ride horses full out across the plains to shoot arrows at bison or hurl spears at elephants trapped in mud wallows.:)
Yep, makes me wonder if that's the beginning of the end. Life is about drive, when the drive is gone for whatever reason the point of it all is gone. You can try to reason it out all you want but hunting and killing only makes sense from a personal perspective, because you want to. It can't be justified in any other way but a selfish way. You can say the shark does need to kill to live, but the prey fish if it could would say well then shark don't live.
Many things affect drive. In nature hunger makes it kick in. Wilder beasts can be in close proximity to a lion when they know it's satiated. In younger animals drive is more of a constant thing. A lot play is used to get rid of the excess. Older animals play less. It also varies from individual to individual. There are guys in whom it runs really strong, they actually don't feel well if they don't take a life from time to time. I'm not very much like that but I understand that evolution made us this way to survive.