The conflict between Tor Vizsla (Death Watch) and Jaster Mereel (True Mandalorians) never was explained in great details beyond some basic informations here and there. The main source, Jango Fett: Open Seasons states that
“Decades ago, the Mercenary group known as the Mandalorians fractured, giving rise to a chaotic splinter group. They called themselves the Death Watch and followed a barbarian called Vizsla […]”
what implies both men were once part of of the same group of Mandalorian Mercenaries. A group that fractured due to ideological dispute between Tor and Jaster, so they must have known each other to some degree.
It is hard to tell how close - if ever - they were once before their different ideologies clashed for good but I find it intriguing that despite the hate and ongoing war, Tor (and even his second-in-command!) called or talked about Jaster just by name.
And this habit did not change even years after Jaster Mereel’s death:
It is really a shame we never got to know the whole story and why Tor, who apparently hated Jaster Mereel so much, called him so casual by first name.
