You will find many quotes from people who have known or worked with Paul who point out his less than desirable characteristics. Ex Wings members are a good start.
But I have never heard of him described as cruel ( I will stick my kneck out and say that's John's department, despite what the Lennonphiles wish to believe. Paul had his own, unique way of being unpleasant). Cold, distant, tight- fisted, businesslike, hard-to-know, callous, domineering......but I can't recall cruel.
I would point out that none of us would stand out too brightly under such scrutiny. Everyone has faults. Theirs are more glaring because we expect greater things from our heroes.