Marcus made a bluff to keep Viktor from fully betraying him. Since he was the father of all vampires, he claimed that if he were to ever die, then all vampires will die with him. Viktor likely suspected that it was a bluff, but he probably wasn't ready to find out. He made the same claim for William to keep Viktor from killing him, that if he ever died, so would all werewolves, including the lycans who were still the vampires' slaves.
If nothing we do matters, then all that matters is what we do. -Angel

Lol, sorry I meant at the start of the movie during the flashback where Viktor and the elders were hunting down William. Viktor said something about protecting Marcus or it was the end of them all.

'Cause Selene killed him, ya silly!!
I don't love her.. She kicked me in the face!!