I love these early episodes as they work through establishing the characters and working out the kinks. Kirk seems off character here, only because he know him so well based on later episodes. There is no talk from him about capturing the creature. It's killed his men and is on board his ship, so all bets are off. Meanwhile, Crater and the creature suggest the very thing that would be entertained in later times: keep it fed, talk to it. It's obviously intelligent and rational and isn't malicious. It was trying to survive as it knew how. Actually, in some ways, Kirk is the villain here. The Man Trap is pretty close to what passed for the average SF show of the day: man vs creepy monster. This feels more like an Outer Limits than a Star Trek. On the face of it, I can see why some critics didn't find Trek's premiere to be any better than the average Voyage to the Bottom of the Sea episode.
Still, with all that said, I do love this episode. It's fun, creepy and is very, VERY serious. The music is so cold and barren, it really contributes to the atmosphere of horror and loneliness of the creature.
I've always figured his botany here is just another one of those personal interests.
Yep, in The Naked Time, Riley complains that the previous week Sulu was trying to get him interested in botany. Sulu is clearly a hobbyist.