Speculation on another site...
Considering the amount of coverage the topic of cancer received in the episode, perhaps Gene discovered during the phone call that Kim has been diagnosed with cancer and his return to the 'life' was undertaken to support her financially. Might be a bit of a stretch, but there certainly was some focus on that malady in the show Monday evening.
- Walter.
Regarding the theme of cancer, one interpretation I had for the end scene was that Gene is pulling a Quint here. Once he found out the guy had cancer, he “became” Walter to Gene. This makes the context of the flashbacks make more sense to me.
And not unlike how Quint ended up, the shot of the cop car pulling up to the garage in the preview suggests that Gene many have went too far.