Matthew 12:34 Brood of vipers! How can you, being evil, speak good things? For out of the abundance of the heart the mouth speaks.
I wonder if you have been called a snake before? I know I have been whilst at boarding school. We used it as a term for calling someone a liar.
If we do not have goodness and mercy in our hearts, then we should not expect to be able to talk and do good and merciful things because whatever is in our hearts will come out in what we do one way or another.
I’m pretty sure the people Jesus was addressing would have taken this to heart themselves and not liked it one bit. It is not nice to have someone point out your bad side when you want to promote a good side.
Don’t focus on the bad but rather try to focus on the good we can do in our lives; good for Christ!