I'm not 100% but I've looked into this attack and it looks probable that the guy finally broke and started killing women due to loneliness and incel status.
He was above average looking, in shape, and tried really hard to make friends and find a girlfriend, he even put himself on an escort website, but due to his schizophrenia people were just repelled by him. If you read up he tried again and again to meet people and make friends for various activities, surfing, language exchange, astronomy etc. etc., as well as dating and escorting. He would stay at backpackers' hostels, which is a top place for lonely people to socialise and strike up relationships. He also frequented strip clubs and brothels.
No friends, no girlfriend, no kids, no job and he realised he was never going to have any of those things, he gave 110% and finally just couldn't take it any more and started stabbing women. The lack of a romantic/sexual relationship was what put him over the edge. He apparently lunged at an armed female cop so he wanted to die. RIP Joel. You're free now.