So why did he live so long and cause so much trouble?

What do you mean, "so long"? Yes, he lived a long time, but he ruled the Empire for only a few months, and that was only because he managed to raise 1000 skeletons and 20 dragons.

By the will of the creators. In general, he could have done a lot and maybe even won this war and defeated everyone, including the demons.

By the will of the creators, the Griffin Empire collapsed along with all its heirs.
He wouldn't have been able to win; you saw it yourself. All his undead were finally buried, and he could only hope to escape. If Zehir's magic killed him, do you think the demons couldn't have killed him? Ashan was lucky that Shio is a prison, not a free country.

But that's just luck! If Isabel hadn't disappeared without a trace, they wouldn't have succeeded. In general, the creators really messed up here, showing a careless attitude towards the plot, because such things need to be clarified in detail.

But where does it say that luck helped? Yes, the creators didn't care about the plot. What difference does it make how Biara got to the throne? The important thing is that she got there. And, by the way, she held onto it much longer than Markel, and she did a hundred times more evil things.

Added 5 minutes later

Then Urgash is a loser, and he is a loser himself.

Then Ashan is also a loser because she locked herself on the Moon. Zehir easily killed Markel, but four of them couldn't take down Kha-Beleth. Draw your own conclusions.