That just doesn't matter anymore. He just wants power
lol it's prob cause capturing a bunch of minion monsters aren't as cool. Tho for the stronger monsters like the wicked treachery, it makes sense why he destroyed the corpse. He's not at the level yet where he can safely capture it or take it down with a clean attack. He can't even cut into the monster so his only way is to attack it's inside, and his only usuable skill for that is an all out destroying one. His priority is to survive and defeat the monster as of now, not to get rich.
also like the reply above, his goal isn't money anymore, it's power.
Now he's rich, anyway his goal had changed for awhile it's not about the money anymore
They had a whole lesson on not destroying the corpse or that capturing the monster will get more rewards but...... our MC is really going all out and destroying them... lol what happened to getting rich