Erica is the most despicable character in this film... (SPOILERS)
All of the character is this remarkable film are both victims and villains. Erica, however, seems to me to be the only true sociopath. For years she knowingly and maliciously infected scores of men with HIV in order to exact some sort of revenge for the abuse she suffered at the hands of her mother's boyfriend.
She actually encouraged these men NOT to use a condom ("Be a man, William. Condoms are for homos.") This indicates that her intention was to infect them and nothing more.
Both Franki and Nate committed hideously violent acts, however these acts were committed in moments of extreme grief after the death of a loved one. Erica's crimes continued over YEARS, and may have killed countless people. Remember, Erica wasn't only infecting these men, but their wives and girlfriends and Lord know who else.
"Not fare well, but fare forward, voyagers."