OMG, what is wrong with you people! Blaming Mitsuki for the accident Haruki (or whatever) got into!
Did you ever had yourself in an accident or lost someone dearly, or wished an event never happened!? They weren't PLANNED, they were just happening in the essence of time, Mitsuki NEVER knew it would have happened to Haruka, she didn't planned it, and she certainly didn't want Haruka to be smashed and with that stupid truck!
The only thing I didn't like about Mitsuki, as Echoes said, is she slept with another man, which is their friend which makes it worse! But I find that 'friend' at fault, he knew Mitsuki was at mental retard state, so he shouldn't have agreed! Men are such lustful creatures, really!