優しくする、優しくいる

Made a mistake during class this week that I just want someone to be kind without a 対象 to be kind towards to, then I should use 優しくいる.

優しくする

to act kindly toward someone
→ more concrete / action-focused

弟に優しくする。
I treat my younger brother kindly.

to remain/be kind as a state or attitude
→ more abstract / emotional / poetic / self-reflective

どんな時でも優しくいたい。
I want to stay kind no matter what.