Was it because he couldn't stop Neeson from killing the son? Was it because he fled the scene and didn't at least try to get revenge on Neeson and his son? That was never fully explained.
Sure, you figure people in that line of "business" are irrational and will blame anyone who happens to be near the victim they care about, but at least some explanation of the specific reason would be helpful.
I think Shawn assumed the guy didn't protect his son as he should.
Maybe Danny and the other guy should have attacked Liam's son together. It seems the other guy just got afraid and didn't do anything. So, I assume he got killed because he was incompetent.
I took it as Shawn knew that his son had a stupid idea, and his stupid friend helped his stupid son play out that stupid idea instead of trying to convince him not to do it, but instead you both went for the stupid idea to raid the house, which ended up getting the son killed. So Shawn felt the friend had to pay for it too.
Good girls go to heaven, all the bad bitches come with me
IMO here is another perspective (in addition to what Vabrown81 and others said here): recall the scene where Shawn explains to his son the history behind his refusal to open the heroin/cocaine drug business ? then there is another scene where shawn looks at this son's childhood picture and contemplates when he "lost" his son ?
Shawn's son was disappointingly a drug addict and he either secretly knew about it or suspected it, Danny's friend was obviously a drug addict as well, Shawn killed him to cover-up his son's embarrassing drug habit.
IMHO, Shawn knows his son's friend/partner knows what really happened and it doesn't match the story he wants put forward, blaming Michael and Jimmy for everything.
Mostly because his son got into a stupid drug deal without thinking, and the friend was obviously a low life hanger-on.
Ed Harris expressed earlier how much he hated the drug business and the ravages it causes to people close to you, so he's blaming (in part) the son's friend for helping br a part of his son's death.
The reason was the loser was into the drugs an working on the deal with his son. Plus he escaped alive while his son was killed. Plus he was pissed. Plus he was ruthless.
I would say my memory is not what it used to be. But I don't remember what my memory used to be.
A lot of other opinions here but my conclusion is that Ed Harris' character was just an irrational dick. The fact that he took his son's killing as personal and not business and couldn't come to terms with Liam Neeson's need to do it shows this clearly. It's amazing that he was able to last so long in the underworld without getting whacked. He needs a lesson from Vito Corleone.