Taiga is not explicitly and intentionally malevolent, but he's for sure a certified asshole. Casting is inherently a bit cruel. Depending on the production, you will reject hundreds, thousands of applicants, and only make one person happy. Just like it doesn't have to be a big deal — just see how many super famous actors got rejected from which movie, it's sometimes even hilarious.
What's unforgivable is to have made that rejection so public and use it to get fundings and plugs for a more qualified actress. And given how much it could have meant for Emi and what she fought for, it stung.
I guessed that she was bitter about her former run-in with fame and wanted to take it or safeguard others, but I didn't expect she mainly wanted to reclaim her shattered sense of self-worth.
This director mf better stay away from Honjou.