Doubtful they ever 'owned' that pos. The attacker may have been a sympathiser in his lunatic ramblings online because of his special interest but I kinda can't imagine AFD would have welcomed this nutter, as he represents what they oppose. It's been reported that he wanted to work with them on a project but that hi was turned down.
I don't think you need to worry about AFD supporters, this guy's actions will have increased their popularity 100 fold. The AFD leader seems scary and fanatic, or maybe just an opportunist but unfortunately is likely to become the next Giorgia Meloni. elections in less than 2 months.
I wouldn't be surprised if AFD gained significant support in places like Saxony-Anhalt and other regions. This event will have left a profound and long lasting impact in the region, they will not forget nor forgive.
People won't focus on the attacker being (allegedly) anti muslim, they'll focus on the fact that he's an immigrant and that he attacked the country that gave him a sanctuary in the most heinous way.