Prime Suspect - Gascard DePuis' Mansion
There are a lot of shades with varying health, and a rage demon. You need to get control of the battlefield quickly. Use a large AOE attack to clear out some of the shades and make it easier to take on the rage demon. It’s a basic grind once you can get a little breathing room. The rage demon is strong, but it doesn’t have much in the way of special attacks. If you’ve got a few potions or a good healing spell, then you’ll be fine.
Once the area is clear, move up and start exploring the rooms a bit. You should just run into a group of four shades as the only other enemy. If you made it through the first group, you shouldn’t have any trouble with them though.
The main bedroom has a chest with woman’s clothes. You can inspect it quickly if you want. The main path is off to the side though, where you’ll find Gascard dePuis and his hostage. Things are a little complicated though. If you talk to him, he’ll explain that he’s just setting up a trap for the real killer.
If you talk to him for a bit, the woman will run off and escape on her own. The real choice is whether you’ll let him go or not. I’ll just cut through this quickly and say that he’s telling the truth, in a way, at least. Even Aveline partially believes him, so this isn’t too surprising.
The friendship/rivalry aspect seems to just come up as preventing a negative. You’ll suffer a bit for letting him live, but it’s not a huge deal. Aveline, Isabela, Fenris and Merrill will not be happy with you if you let him go, although it seems to vary a bit depending on whether they like you already and to what degree you question him. The hits aren't big though. Just 5 or 10.
On the practical side, killing him now cuts off some options in the follow-up quest. It doesn’t necessarily make it more difficult, but it makes it more narrow. Note that if you change your mind, you can frame him as the killer anyway in another moment.
If you want to kill him, then you can do so. You can also let him go. It’s your choice.