I kill Connor too. But the difference is that Connor is a child who has never had proper training. Anders is an adult who has (the Harrowing). Children typically have lighter sentences than adults for that same reason. An adult knows better, and Anders knew better than to allow Justice inside his body. He had every single warning in Awakening and chose to ignore it because he is a fool. Justice told him that demons seek out mages and that he had no such desires. He should have known that the moment he had the desire to enter his body he was a demon or close to becoming one as he was being perverted from his nature and should have refused him. He can cry about the injustice of the Circle as much as he likes. But ignoring the lessons of his Harrowing placed everyone's lives in danger.
But Connor never had the chance for any training due to his mother keeping him from the Circle, so his getting possessed was hardly his fault as he had no idea of the consequences of what Desire intended to get in exchange for helping his father. It also didn't help that the mentor she got to teach him in secret was a blood mage known for making idiotic decisions and was also hired to poison her husband. I'd say that the cacophony of idiot decisions from the adults around him, means that Connor deserves the least amount of blame for getting possessed.
As for Justice, he was already in that situation already, as he'd been forced into Kristoff's corpse against his will and he didn't know of any way to exit that vessel except by jumping into a new host. As Anders said, he fully consented to have Justice use him as a vessel to help his friend, rather than be either tricked into it or agree for power, as in the manner of demons. Considering that Riviani Seers regularly take Spirits into themselves with little problems, it's not like it can't be done safely, if you know how to do it properly (which Anders admittedly didn't).
Sebastian doesn't look worse. He has every right to search for Anders and those who aided in his escape, if they did. I think he went about it the wrong way, unless he tried to go through Bran first and was denied. If so, then why? Do I think he still had a right to invade? No. But that's what spies are for. He should have sent some into Kirkwall to dig around Anders's old place, mage underground et al to find clues as to where Anders might be. But at the end of the day, I could care less what happens to that dump anymore, but I do help Sebastian's relief efforts solely to solidify the alliance . I'm just glad Hawke is no longer there.
But what right does Sebastian even have?
As a Prince of Starkhaven, he has no authority as a foreign national regarding how the city-state of Kirkwall runs itself. As a Chantry Brother he has no authority in hunting down a rogue apostate as this is the purview of the Templar Order, who are currently engaged in shenanigans elsewhere. Bran was well within his rights as acting Viscount to tell him to take a hike and go be boring back in Starkhaven (after complimenting him on his familiar voice).
Furthermore, spies would be useless as the Mage Underground was destroyed by Meredith in Act 3, which is part of the reason that Anders fell so completely off the deep-end. With Hawke and everyone having left Kirkwall (save for Aveline), there is no-one around who he can question for aiding and abetting Anders, so his invasion has absolutely no point.