Magnanimous but in a conceited manner
Solution 1:
Someone behaving in a patronizing way might be what you are hoping to describe. However, they wouldn't be acting magnanimously any more.
Also, many forms of patronizing behavior have nothing to do with forgiveness and generosity central to the common meaning of magnanimous.
patronize at Oxford Living Dictionaries ˈpātrəˌnīz,ˈpatrəˌnīz verb
gerund or present participle: patronizing
1. treat with an apparent kindness that betrays a feeling of superiority.