en_tn_condensed/mat/12/34.md

You offspring of vipers

Here "offspring" means "having the characteristic of." Vipers are poisonous snakes that are dangerous and represent evil. See how you translated a similar phrase in Matthew 3:7.

how can you say good things?

“you cannot say good things.” or “you can only say evil things.”

out of the abundance of the heart his mouth speaks

“what a person says with his mouth reveals what is in his mind”