When Morgen died, he did so to:
Send Lucifugus back to the underworld, destroying the relic that tied the devil to their world
Nacht summoned Lucifugus to try forming a contract, which would put the devil under Nacht's control.
But to establish that contract, Nacht would have to fight Lucifugus 1-on-1 and win.
Nacht couldn't do that. He knew the moment he summoned Lucifugus.
If Nacht loses, he dies. The only option to live is to win, but he can't do that either.
So Morgen made a third option: send Lucifugus back, and stop the contract from pushing through.
Nacht didn't know what the consequence of destroying the relic was. He went in, thinking he could beat the supreme devil, just thinking he had to surpass his limits—and that drive buckled underneath his instincts to live.
Meanwhile, we also don't know the extent that Morgen knows of Forbidden Magic and rituals. The only hint is what he says as he dies,
He just knew they were working with the magic. If Nacht didn't know the consequences despite spending his time studying and practicing, I doubt Morgen - who stayed out of the family practice and tradition - would know.
But Morgen did know that Nacht had four devils. He might've watched Nacht finish his fourth contract in disguise, since Nacht was in the summoning circle before trying Lucifugus. But also, their father literally says Nacht contracted 4 devils, right in front of Morgen.
Morgen intervenes when they try for a fifth, high-ranking devil.
Below, I think "Any more would be-" is referring to Nacht having more devils. 4 is a lot already, since everyone usually just has 1; usually a single high-ranking one.
A high-ranking one, on top of 4 other devils - 4 mid-level ones at that - is too much.
"Any more [devils] would be-"
So Morgen doesn't know much of Forbidden Magic. But from watching his family in disguise, probably as Nacht contracted another devil, he could've pieced together the purpose and role of the devil relic. Maybe he pieced together the whole contract process.
So he destroyed the relic.
Lucifugus was too much to defeat, and Nacht had to fight that alone—but not if they sent it back.
Even if Nacht won, what would happen to him, having so many devils? If one loses the devil-binding ritual, they become a monster. What about someone who has too many devils? Wouldn't their humanity start to lose out to their numbers and power?
By getting rid of the fight before it could start, Nacht couldn't win or lose. There would be no consequences except on Morgen, who didn't know the backlash, or if there was any at all.
There are probably other reasons one can come up with, but for me, the main one was to save his brother