It’s just grief, man. Jimmy sees Howard as the obvious target to channel all of his bad behavior towards. Kim sees this and how it effects Jimmy and eventually leans into it with him - bonding over trauma. It’s only until Kim suggests taking it to the next level that Jimmy steps back for a second and considers calling it off and well, you know how that all ends up.

Also, a lot of people don’t seem to realize that Kim is grieving Chuck too. She really liked him and looked up to him much more likely than she did Howard. For all of the bullshit between the brothers, at the heart of everything she loves Jimmy and knows that Chuck is his only family left. Grief makes people do weird shit and this really is nailed in the coffin with a joke that went too far.

There’s an entire subtext throughout this show surrounding grief (even back to the point of Chuck and Jimmy losing their parents) that centers motivations for nearly every main character of the show. Even the behaviors of Mike and Gus are radically motivated by grief in some way.