I’d call them racist for all the shit they gave Thorax, just cause he’s a stupid changeling.

@Trilby Pony  
That, and she was probably remembering what happened with Trixie back in “No Second Prances.” Did Twilight really want to make the same mistakes again with Thorax?
 
She allowed her own past with Trixie cloud her judgement, and she nearly let it happen again here with Thorax, hence why while she may have been angry at the start of Spike’s song, we see her here already softening as she realizes she’s repeating what happened with Trixie again with Thorax and knows Spike is risking everything to prove Thorax is good.
 
How could Twilight remain angry at Spike for apparently betraying her and Equestria when he’s putting so much on the line for Thorax? They’ve been together for a long time, and Twilight knows Spike enough to trust him and put her own past with the Changelings behind her, even if the others are not as easily swayed as she was at this moment.

I’m pretty surprised they accepted him so easily, I mean, what if spike was another changeling? That would’ve been an interesting plot twist :D

@09MurphyM  
I liked that, myself. She actually started thinking “Spike’s usually a pretty good judge of character. Maybe we should give him a chance…”

I like twilight’s moment of realization when she realizes that if spike says the changeling is nice, he must be nice.