Basically, Its really a long debate but I am giving you two reasons.
1. It hurts Muslims, its untolerateable and unbearable.
2. This one's a bit complicated. Allah/God loves Prophet Muhammad (S.A.W.) a lot. God is extremely angry to those who make photos especially of Prophet Muhammad S.A.W., His beloved, the best from all Creations. Prophet Muhammad (S.A.W.) was extremely kind and Merciful, He never wanted that people may burn in hell but if someone creates His (S.A.W.) pictures, He burns in hell.
So, Prophet ordered at some occasions to kill the blasphemers because if one blasphemer is killed, Others don't commit blasphemy and they get a chance to be saved from Hell torment