Why you kill them immediately:

I died. I respawn back at my bed. I knew, but somehow I hoped that mr. therzinosaur by my home left (thank the Lord our God that I have two homes). It didn’t. It saw me as soon as I stepped outside. I ran inside. I saw claws and it said ‘stone foundation 19874/20000’. It kept going down. I panicked for a few minutes. I had no idea that I could just kill it slowly by hitting its claws every time they appeared. I did that. After, like, 25 minutes, it said ‘Demuck (my name), level 21, has killed a therzinosaur, level 1’.