I can't say that the police knew full well why Erickson was doing what he did -- they didn't delve into his mental state. As I recall, Erickson told someone that he was involved in the crime and that person then called the tip into police. And the rest is tragic history. Tunnel vision I buy; I really don't believe that cops want to put someone innocent in prison, I really don't. Yes, it happens, we know it does. They convince themselves they have the right person and then of course that person must be punished.
When you have someone who confesses to a murder and does so without coercion (remember Erickson implicated himself first and that was to someone at a party and he brought the topic up), it's easy (and frankly lazy) to take it at face value and race ahead.
I've never heard of a homicide detective in the last couple of decades who specifically wanted to get someone who was absolutely innocent of a crime, knew in advance they were innocent of the crime, but they wanted to put them away forever just because... and I don't see that in this case -- the blinders were absolutely on and they simply did not remove them even when the evidence didn't match the suspects.