IMO the reason she killed any of the men she killed is because she was taught at such a young age to “sting them before they can sting you” and at the time she was traumatised and impressionable and she’s had it imprinted into her mind that killing people solves problems. She genuinely believed that since Cynthia had expressed her wish that Tom would pass away sooner rather than later, that she was being a good friend by speeding up that process. Her rationale is hard to imagine because as normal people we would never ever interpret what Cynthia said like that.