uhm... dunno it certainly made more sense on coppola's dracula, with all the "your beloved have committed suicide so she will burn in hell for an eternity" and all.
still, for how i understood the plot. it's not exactly mathias turning evil.. or more evil of what he already was. maybe he never give much consideration at the price to pay to fulfill one objectives. therefore using sahra to make leon kill walter wasn't such an unnatural thing for him to do, he just never had the need to do it before.
and he was angry at god for forcing humans to endure the bitter taste of mortality, such as his beloved elizabetha did, so he turned into a vampire as a way of saying Fu** you god, i'm out of that rule.
there isn't any record that he used his power for greater generic evil (just generic heresy) until Lisa was killed.... by humans, so i guess its kinda understandable?
also before lisa death, there was various not so evil people who cared for Drac, such as Lisa, Alucard, uhmm.. this guy Hector.. etc.