How do I get back on my players?

For Context: 2 of my friends wanted to try out DnD, so I set up a homebrew Campain with NPCs and a good story thats easy to understand. 2 Sessions in they killed 3 Allys and Abandoned one in a Colosseum with a dragon after not taking the clearly intended path and breaking through a wall to skip a section where they would get the loot to fight properly. I think I should make the Abandoned NPC a Villain who wants revenge. Any Ideas how to do that? My Plan is to make the NPC defeat the dragon to become a boss after half of the campain.

If anyone wants the Story and worldbuilding I can explain it.