Let's say Betelgeuse exploded 550 years ago, and I happen to be looking at the star at the exact moment the light/EM reaches the earth. Would it be bright enough to cause an injury? If at night, would it make shadows on the ground next to me?
Found this source quickly that says more like a half-moon, but it's all just estimates really. https://www.astronomy.com/science/when-betelgeuse-goes-super...
I think it's safe to say it would be unlikely to cause any damage to your eyes if you happened to be watching.
There are a couple of other factors. Supernova "light curves" only reach their maximum gradually over several days. Also, your eye doesn't focus an arbitrarily small light source to an arbitrarily small spot on the retina. Instead it's something like an "airy disk". At some point, the relationship between apparent size and the size of the (blurred, "diffraction limited") image on the retina is appreciably non-linear. I think it matters in this case, because Betelgeuse is a lot smaller than the eye's "resolution" of about 1/60th degree. So its light is concentrated that much less (squared). In a supernova, the "size" of the star increases over time, though maybe not enough to matter, even for Betelgeuse.
So your eyes would have time to adjust.