I think it's more nuanced than cowardice. Disillusionment, bitterness, frustration, etc. Most people change over 30 years. Expecting him to be the same and then being angry when he's not is pretty dumb, imo.
But the bolded is fundamentally wrong, isn't it? He briefly considered murdering him, but never actually attempted it. And was he really innocent? There's an interesting ethical discussion to be had there about preemptive action towards evil. The Roshomon-esque retelling of that moment is one of my favorite parts of TLJ.
The central characters in this saga remained the same more or less, Han and Leia aged 30 years and haven't changed from what we've seen.
Going further back, Obi-Wan lost all faith and hope, all his friends and allies pretty much killed by his apprentice in the same day, and forced into exile for decades.
However when he got his chance to strike at the Empire, he took a chance without question or hesitation.
I find it unbelievable a student who sought the redemption of Vader would be driven to try to kill his nephew, regardless of the passage of time.
Luke placed blood/family/friendship over ancient feuds and ideology.
in ROS would be crying foul over people shytting on her character if she actually got a lot of screen time in this last movie 
