The words murderess, murderous sound the same but have different meanings and spellings. Why do murderess, murderous sound the same even though they are completely different words?
The answer is simple: murderess, murderous are homophones of the English language.
A woman who commits murder. See Usage Note at -ess.
Capable of, guilty of, or intending murder: a group of murderous thugs.
Characteristic of or giving rise to murder or bloodshed: murderous mistrust.
Informal Capable of devastating or overwhelming: a murderous exam.
Definitions from The American Heritage® Dictionary of the English Language, 4th Edition and Wordnik.
Homophones (literally "same sound") are usually defined as words that share the same pronunciation, regardless of how they are spelled.
If they are spelled the same then they are also homographs (and homonyms); if they are spelled differently then they are also heterographs (literally "different writing").