Is there any consequence for me for not verbally denouncing a statement of disbelief uttered by my cousin, knowing that I condemned it in my heart and wished for his death, and was it obligatory for me to leave?

If you were able to denounce with your tongue but did not, and only denounced with your heart, then you have sinned and repentance is obligatory upon you. However, if you were unable to denounce with your tongue, then you have done what was obligatory upon you by denouncing with your heart, and there is no sin upon you. And it is not obligatory to leave the place unless the objectionable act continues and you are unable to remove it. You must denounce with your tongue with wisdom and good counsel, for punishment is the prerogative of the ruler.