So great ! People love comparing Viserys to Joffrey, but honestly, Viserys has a horrible backstory, and he never had anyone to be there for him.
No one loved him and him seeing his sister be admired and loved by so many people made him feel like shit.
If I was in his position I probably wouldn't end up being a nice person, either. But Joffrey? He's just a spoiled brat.
Viserys is truly a tragic character. Daenerys said he used to be kind, and sweet. Exile and constantly running from assassins changed that. Up until the night of his death all he ever wanted was to go back the westeros and be king. That doesn't excuse his madness (like father like son) but hopefully sheds some light on it.
He is still the reason why Dany is still alive because he protected her and was her only family while she grew us.
But Dany also killed their mother when she was born so he could never look at her without remembering the live they might have had.
He hates and loves his sister, just as he hates his own weakness and powerlessness.
And that's a tragedy