He had "borrowed" the power. He felt as though his life had reached its climax, as though it had reached its peak.
Unfortunately, he had let those last few get away. How stingy could they be? They wouldn't even give him a little power. Fine, they wouldn't lend it to him, but they had cursed him out and thrown him out too. Once he was strong enough, if he didn't clean their dimensions out completely, that would count as him being kind.
Forget it, forget it. No use getting angry.
"Ancient One, how are the brother and sister doing? Have they been well lately? They haven't torn the Sanctum down, have they?"
"They've been doing well lately. They have good control over their powers. You used to wreck the halls far more often than they do." It looked like the Ancient One was quite satisfied with the two of them, but why had she started taking shots at him?
"What are you saying, Ancient One? I'd only just arrived here back then, and I wasn't familiar with the place. A few little bumps and scrapes were perfectly normal. I was very well-behaved too, you know."
"If I remember correctly, you still haven't returned the Sling Ring you took, have you? Back then, you blew up more than half the library trying to combine your own abilities with magic. Wong nearly went mad. He wouldn't let you within fifty meters of the library for more than three whole weeks."
Now that the Ancient One mentioned it, Rimuru remembered that the Sling Ring he had swiped was still in his Stomach. He didn't seem to need it anymore. "I was just keeping it as a souvenir. I'll put it back in a little while. And that explosion was an accident, all right?