I thought that the line was 'but sentimental boy is my nom de plume', which I would take to mean as he's being sentimental because he loves her, but it's not really who he is, and that's part of why he has to leave.
Also, it being his nom de plume kinda suggests that he's been in similar situations before and so he knows it will end badly, and the only way to save her would end up killing him. So even though he knows what to do, he's still around, kidding himself into thinking it's still okay, and hoping something will come along to spare him :)
I thought that the line was 'but sentimental boy is my nom de plume', which I would take to mean as he's being sentimental because he loves her, but it's not really who he is, and that's part of why he has to leave. Also, it being his nom de plume kinda suggests that he's been in similar situations before and so he knows it will end badly, and the only way to save her would end up killing him. So even though he knows what to do, he's still around, kidding himself into thinking it's still okay, and hoping something will come along to spare him :)