Alex is heavily addicted to, I think, cocaine. Has been in & out of rehab many times, and that morning Maggie tells him Paul has found a new bag of his drugs. Alex is broke. No hope of buying more. That may have driven him over the edge to murder.
@RKirbySC The morning of the murders, Maggie told Alex that Paul had found another bag of drugs Alex had. Alex had been in and out of rehabs almost too many times to count. Now he is broke and heavily addicted. It is possible that the threat of not having his stash threw him over the edge.