The road to hell is paved with good intentions. Magnus had good intentions, but gosh-dangit to heck, he could and should have used his noggin just a little, instead of encountering a psychic barrier and thinking "I'mma psychic kool-Aid Man through this shit!".
Where I feel other people screwed up, was the Space Puppies having such a murder-boner for the Thousand Sons that they didn't think to ask, for even a moment, why Big E apparently just changed his mind at random and asked for a genocide instead of the original order of "bring Magnus to me so he can answer for his crime", something Magnus was ready and willing go through. Had it been Girlyman or Rogal Dorn, I feel they would have at least taken the time to look at the updated orders from Horus and gone "Hol' up, this warrants at least verifying".
Perfect example of there was nobody in the right in that situation.
Yea, I'm not an apologist for Magnus, much as I like the Thousand Sons, but he absolutely got shafted because of Leman and his puppies being such hypocrites.
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*cough*
Now if I got something wrong, feel free. I am but a mortal man and my knowledge often gets jumbled.