There's a weird amount of satanic beings in Baldur's Gate 3. Between all of the infernal beings that you can make a deal with as a warlock, Raphael who you meet with throughout the main story of the game, and Yugnir, you'd be forgiven for thinking they're all a lost cause. But there are actually benefits to siding with Yugnir in Act 2.
Should you help Yugnir or kill him?
If you help Yugnir during his quest in Act 2, then he'll show up later in the game to help you out in a critical moment. However, this will also lock you out of an Astarion romance and will prevent you from finding out the secret about the markings on his back. This will also have a deep effect on later potential questlines in the game, so be extremely wary of this before you attempt it.
However, if you choose not to help Yugnir then don't fret. You can still persuade him during the aforementioned crucial fight in the game to help you with a DC Persuasion check. But to fight him here, you will need to either fight his followers, his displacer beast and Yugnir himself, or you'll need to pass three different high Persuasion checks to get Yugnir to kill his followers, his beast, and then himself.