I don't believe he is killer material though.
My opinion is that Brueckner is an archetypal predator who goes for weaker people than himself. I'm not sure if that would include males, but if the opportunity presented itself he probably would just for the fun of it and because he could.
His history with females - for example - his girlfriends, doesn't come over as a happy one for them; and the torture and degradation inflicted on his older victim says it all. Quite a variable age range there from abuse of the young right through to abusing an elderly person - and usually with some monetary reward in it for him.
Given the phone evidence of the third and his tout de suite departure from Luz, I think puts him in the frame as instrumental in taking part in Madeleine's abduction; what was done with her thereafter is less clear.
Perhaps if the PJ had given more credence to following up physically possible sightings of Madeleine regarding time or place we would have a more complete picture or even Madeleine being returned to her parents within days.
I think Brueckner is perfectly capable of killing an innocent little girl and the German prosecutors certainly believe they have evidence to support their conviction he did.
My blood just runs cold every time I read about his dark web ramblings describing his fantasy. I think he would have been perfectly capable of carrying that through to completion particularly if there was money to be made.
I sincerely hope he never did. But yes, I firmly believe he has no conscience and would be capable of murder.
The tabloids are presently running with a story that he killed an ex-girlfriend when he was drunk, I think his history of violence towards females doesn't make that improbable.