The deed that Hedda refest to here is what she thinks is Lovborg’s suicide.
Hedda, while talking to Brack, characterizes Lovborg's suicide as "the last great act", claiming that she admires that "he should have the will and the strength to turn away from the banquet of life".
She believes that Lovborg was brave upon deciding to take his own life so young, as the quote clearly states ("a deed of deliberate courage".)