It appears that this blew up and was solved before I came online.
While I think Klaus grabbing Heinrich before he fired is a good idea. I am not sure Mortus would agree, I know things that Mortus does not know. Given the information that was available to Heinrich at the time, and more importantly the information that Heinrich did not have, I think it is not an out of character action for Heinrich to attempt to shoot him. You must remember that
1: Our aim is to disrupt the ritual, prevent it from happening.
2: A bounty hunter would normally be bring in his prey dead or alive and as most bounty hunter's prey is dangerous it is normally easier and better to kill them first. Also it is important to note that the medieval (and warhammer world) life is cheap and not that important. Most people knights, police, bounty hunters and stuff would not think twice about killing people.
3: We did not know that ye-man was a wizard. There was no information to suggest that he was. We know that Teugen is a wizard or at least has magical ability but there was nothing I remember to suggest that the other guy was.
4: Heinrich has been very successful with his bow up until now, therefore his confidence would be high that he could take a good shot, therefore it is more likely for him to do so then normal.
5: We believe that the ritual needs all seven people to complete.
6: Heinrich is scared of demons and more so of the type of demons that these guys are going to raise.
Therefore it is not unreasonable for him to consider that taking out yeman and hiding the body and waiting for the next guy is a good idea. I also think that people have too modern a view of law enforcement. Being friends with ye-man is good reason for the law enforcement to look into his actions and as soon as they find he is even interested in magic they may burn him just to be sure. Being connected with somebody that is going to raise a demon and being interested in magic is normally enough to get you shortened by a head. Sir Martin and the baron are not going to be worried about legal niceties.
On the whole I think we are better off not taking a shot at him and having Heinrich killed but I don't think that it is a OOC move for him to make.