"We're done talking about the (June 27) debate. It's time to put Trump in the bullseye. He's gotten away with doing nothing for the last 10 days except ride around in his golf cart, bragging about scores he didn’t score…The fact is, he's not answering questions."
After the debate Joe Biden had to answer many questions about his performance. Some of the people in his own party have even talked about him stepping down in the aftermath of the debate.
Joe Biden was the in the "bullseye".
When he said its time to put Trump in it, he was talking about shifting focus away from his perfomance at the debate to trump... not murder him.