How come the Guardians died so fast and why did Omni-Man do it?

So I just finished season one and can't wait to start season two! However, I'm a little confused about some things.

I know Omni-Man was set on conquering the Earth for Viltrum and I think it's explained that he killed the guardians because he thought they were a threat to his plan but I'm a little confused.

It is stated several times that the guardians were EARTH's greatest defenders, not the country or anything but Earth. That is insane. I also assumed they've been doing this for a few years, so they should have some experience fighting dangerous threats, like Omni-Man? Why was it so easy for him? I know he is a Viltrumite but he took them down in a matter of MINUTES. I feel like Red Rush and Green Ghost definitely could have helped more or something.

And it's not like it was a 1v1, it was the entire group. If it was so easy to kill them too, what was the point? It's not like they would stand a chance later.

Maybe I am overthinking this because I just binged the show lol but I'm very curious.