Sure, this sounds reasonable to me. I wouldn't turn my back on him once the eggs were saved from these double agents, however. If I were the DM, his agreement with the PCs wouldn't extend beyond dispatching the cultists in the Dragon Hatchery. He does what he wants within the limits of his code and the letter of the agreement (not the spirit) with the PCs. Once the double agents are dealt with, he tells the PCs to leave immediately or be slain by his hand.
Having an enemy reveal his or her motives is also reasonable in my view. It's a classic trope that villains monologue their intent anyway when they think the protagonists can't do anything to stop the plans they have put in motion. So, sure, why not?
For suggestion in general:
DMs, if a player has his or her character cast suggestion on an NPC/monster and the monster fails the save, interpret the suggestion and the effect on the NPC/monster that follows in the most favorable way to the player you can with an eye toward making the interaction fun and add to an exciting, memorable story. If an NPC has cast suggestion on the character and that character has failed his or her save, then offer the player Inspiration for playing true to the suggestion, again, in a way that is fun for everyone and adds to the tale being created during play.
Players, if you are planning on using suggestion, don't abuse it. Use it in cool ways with reasonable suggestions that fit the scene and the target well. Make it easy on your DM to say "Yes" to your ideas. If you're the target of the suggestion spell, agree to do whatever reasonable suggestion is put forth and ask your DM if you can get Inspiration for it if you play it up well.
If everyone involved keeps these principles and the goals of play in mind, then you should have no issues with this spell at all.
