Why is Love worse than a normal curse? When Homura's Soul gem becomes tainted it fills with swirling colors that Nagisa calls worse than curses. Homura then says shortly after while reality is being recreated that what transformed her wasn't despair but love.

Why did Nagisa think Love is worse than a normal curse?
Answer:
Nagisa never said nor thought love is worse than curses. Both Nagisa and Sayaka couldn't explain why Homura's soul gem had those colors. Homura bore way more negative energy within her then ever seen before. Way more than a curse ever had borne.

Side note: Homura claims that all this negative energy comes from her love for Madoka due to several events:


Being unable to save Madoka the way she wanted: by preventing Madoka from becoming a Puella Magi
Losing Madoka (again): Madoka vanished from visibility when she became the creator of the new universe, a.k.a. Madokami
The desire to bring Madoka back (as a plain human)
The idea of being able to meet Madoka(mi) again when Homura's soul gem becomes completely black
The selfish act of taking Madoka(mi) for herself and thereby breaking the universe