Hanlon wanted to own the sports franchise, when he's on the phone with Eric he tells him they have a meeting to buy a hockey team and Eric says he doesn't want a hockey team, "Hockey is for penguins" he says, then when Hanlon says they can make two million in the first year Eric says "who needs it". Eric had no ambition and Hanlon wanted it all, that's why he killed him.
I also think Hanlon was having an affair with Eric's wife. He went to the airport to pick her up and she ran into his arms. Then while the police were trying to find clues on the tape she is holding and squeezing Hanlon's hand, and then told them all to get out of her house as if she didn't want them to solve the murder.
Also Eric told Hanlon he was going to fire him on of these days, he said next time you call me kid you've had it. Hanlon wanted to get the team while he had the chance. And I think sure Eric's wife was in on the whole plan.