But... he doesn't respect him
Education , common curtesy ect

. I don't see any reason why he should not shake his hand , ronaldo is not a saint , but he hasn't killed anybody either from my knowledge , i myself don't like his attitude that much , but i don't think he has ever done something that grave to not deserve a handshake , also considering that he was the one who elongated the arm for the handshake . It's like a call for peace , aknowledging your opponent , accepting that what has happened in the field remains in the field ect .
Not a classy act by guardiola