the bit I liked most about the movie was mordo's character development so lemme try and explain the way I saw it all,
his whole existence and faith was rooted in following the rules so that earth is safe from evil. Avoiding forbidden magic is a part of his faith. Then he finds out that the ancient one was using forbidden magic and THAT is the reason/opportunity for mads mikkelson to become the evil character he's become and wreak so much havoc. So after strange saves the planet by using forbidden magic mordo says again that 'the bill comes to you', just like he did when he learned of ancient one's use of forbidden magic, ie that strange has just caused a cause-effect chain that will punish him/earth for his use of forbidden magic, just like mads mikkelson was the effect of ancient one's using forbidden magic
And so at the end mordo is beginning his crusade against all wizards, who to him cannot help but pervert reality... and thus cause cause-effect chains that end in negative consequences
I realize the last bit is a bit of a leap from forbidden magic -> altering reality but I'm OK with accepting that as his motive and am excited to see him as the next villain
I'm not actually in the know about dr strange cannon so maybe there's a better explanation though btw