Well, being in a robotic suit, he didn't have to worry about pain from his stub, and Palpatine was a bit distracted with frying Luke, and didn't see it coming.
Though you'd think that being in a mechanical suit would've been an advantage - super strength, no pain, and he should've been able to move super fast, too.
The explanation that he wanted more power sounds pretty plausible - Sidious obviously didn't teach him the lightning trick (or maybe you have to have human hands to pull it off...I'm not a Sith, so I wouldn't know).
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