I can't think of Reimu as an asshole for now, she's just a poor girl that give her virginity to someone that didn't deserve it, and thjat happens a lot nowdays. At this point, she's just thinking about a way he culd take it back (well, she can't, but...). Yes, thinking about killing Marisa was bad, but I can really understand her despair....
Well it doesn't matter much, she became derranged, the best scenario is both to die, Marisa to pay for her selfishness and Reimu as a mercy death since she's way beyond salvation mind wise.