Yet.. Sanguinius was the one who delivered the blow that caused the Emperor to be able to even defeat Horus in the first place.... Oh.. and he happened to be there with his legion, not on teh otehr side of the universe.
The fact was, Guilliman was the Lord Commander of the Imperium, so it was most defiantely him who ordered the Navy to fire on the Imperial Fists.
And thats Corax, 'Oh I'll just mutate my chapters geneseed and create a bunch of mutants I'll have to euthanise', and Khan 'I'll hide at the back in this Razorback while you guys fight out there' who supported him?
And having 70 -90% of your chapter destory, just defending not being in battle against any tough opponants, after the Heresy... thats really bad leadership in anyones books.
The fact is, the Smurfs aren't very good at anything and got everything they did by sheer luck rather then battling for it.