If force users can shoot lightning from their fingertips, it's not that much of a stretch to be able to make it shoot from a cloud. Who knows, maybe Obi-Wan, Anakin, and Qui Gon combined their powers with Yoda to help him make that lighting.
Either way, my main point is that Yoda is unlikely to abuse his powers in the way that the OP describes. Killing someone that can't defend themselves as a Force Spirit would very likely darken you and prevent you from ever becoming a Force Spirit again, since only Jedi can become Force Spirits in the new canon.
I mean that's a personal explanation you have, but nothing is told nort shown, except that Yoda can invoke huge lightning and destroy real things. Look how Luke was knocked off the tree by the powerful fire.
Yoda stepped up and tried to kill Sidious. Why wouldn't he step up to fight Snoke and Kylo Ren? This does not really make sense and I don't think Rian Johnson has thought any explanation. He doesn't care.