While exact definitions may be difficult, most humans can agree on a general one. We may get caught up in the particulars which can vary from culture to culture, but if you ask humans pretty much anywhere if murder (for example) is acceptable, they'll tell you no. We may bicker over things like religion and customs, but ultimately most people could agree on the more egregious examples of human behaviour. Call it 'evil', call it whatever you'd like, but people tend to recognize it when they see it.

As for negotiation, I'd say yes, if you can convince someone 'evil' to change through reason, then by all means do so. War will always cost lives. It's occasional necessity is a poor reflection on humanity as a whole. Reason should always be attempted first, leaving force as a last resort. Unfortunately, the application of reason generally requires reasonable people and the evil ones almost never are.