AL DM altered the stats on a monster and killed two PC's during death curse.

[Dalebert]

Yes, they absolutely can. The AL DMG specifically says DMs can alter scenarios and monsters so long as they keep the basic intent of the scenario.

No. They can remove monsters and they can add thematic monsters from approved sources, e.g. MM, Volo's, any of the monsters introduced in various campaign books, e.g. crag cats from SKT. Arguably this could result in a completely different encounter. They absolutely cannot make up new monsters or alter monsters from those approved sources. You can possibly increase their hp within the normal range the monster can have instead of just using average. That's about the only stat change allowed.