From what I understand, he wasn't sent there so much as to erase time, but to erase Dracula from time so that Galamoth rises earlier. In the future after Soma's timeline, Galamoth is the ultimate evil now that Dracula is gone. I can only suspect why he did this, either because he hates dracula THAT much? Or simply because something happened related to dracula (perhaps Soma?) that has hindered his evil plans in the future. I suspect that Soma had continued his battle against Galamoth, but no longer for evil reasons.
Regardless, we wont ever find the truth because:
1. Judgement didn't niether sold and wsn't recieved well, so no sequal to continue the plot is likely.
2. Anything regarding time travel has huge plotholes, and castlevania already is filled with them... forget about trying to make sense of it all. It will consume you with evil and turn you into a dampir!