Horst Mahler is a very interesting person. I can only speculate as to why he underwent such a radical transformation, and honestly speculation is not what this subreddit is about. However I can offer some factual insight that may offer some sort of explanation.

Mahler was a very prominent and public leftist in Berlin and the Federal Republic in the late 1960s. Like Ulrike Meinhof, he was older than most of the young radicals (by about 10 years) that he spent time with. When Carlos Marighella's "Minimanual of the Urban Guerrilla" came out in 1969, it seemed to offer a blueprint for Mahler for how a small group of dedicated people could effectively wage war against an imperialistic government (Marighella's book was really geared towards South American countries like Brazil, Uraguay, etc... and it was also fatally flawed in many of it's assumptions).

When Andreas Baader was captured and imprisoned for his role in a 1968 firebombing of two Frankfurt department stores, his girlfriend, Gudrun Ensslin (who was on the run herself, and wanted by police), desperately wanted to break him out of jail. Mahler was proposing a new group of Urban Guerrillas, and this dovetailed with Ennslin's desire to free her boyfriend; the group's first action could be to break Baader from custody.

But, Mahler was a bit of a bumbler; earlier, when they had caught Baader and placed him in police custody, Baader gave the police a fake identity. The police were trying to determine who they had in their custody when Mahler called the police station and said "I understand you have Herr Baader in custody..." And they essentially replied "well thanks for confirming that it IS Baader in our custody".

After Baader was broken out of custody, the nascent group flew to jordan to get training at a Palestinian guerrilla camp... an idea of Mahler's. By the time the group returned to Germany a month or so later, Baader and Ensslin were pretty firmly in charge of the group. After a few bank robberies, Mahler was one of the first members of the group captured by police (mostly because of the ridiculous and obvious wig he was wearing to cover his noted bald head).

While in prison (he eventually served 10 years of a 14 year sentence), he wrote a manifesto for the group; he still considered himself their leader. This enraged the members of the group that weren't imprisoned; specifically Ensslin and Baader; and they had Ulrike Meinhof write a competing, and "official" manifesto; "the concept Urban Guerrilla."

So essentially Mahler was systematically ostracized from the very group he founded, he was a man alone.

Again, I cannot speak to what caused his conversion to the right through the 80s and 90s. But I can say that he effectively had no comrades from the left anymore; and his reputation was as a bit of a bumbling doofus. So I think it would plausible to imagine that he gravitated towards an ideology where he could again be respected in the narrow circles of people that supported that particular ideology.

Major Sources/Further Reading: Aust, Stefan. "Der Baader-Meinhof Komplex" 1988 and www.baader-meinhof.com