Some RIR roosters can be mean.
I have only had 1 mean one, we called him Diablo, so we gave him away after he spurred my DS. He was a banty roo, but he could jump. I did notice that he did not start getting an attitude until he was about 1 1/2 yrs old. I now have one of his sons who is very gentle with his hens and with us.
I have had 2 RIR standard roos that have been very gentle.
I don't know why some are mean and some are not, even if raised in the same environment and gets the same amount of attention.
Sorry I don't have a definite answer, but I wanted to share my experience so you know you are not alone. There are many chicken owners who don't particularly care for RIR roosters, because they can be aggressive.
Good luck.