What is a quote that shows how Victor was filled with grief and sadness due to the deaths that the monster caused, or that the deaths weighed very...

While he witnesses the trial of Justine, who is accused of the murder of little William Frankenstein, Victor knows that she is innocent and that his brother was killed by the monster.  He says,

"The tortures of the accused did not equal mine; she was sustained by innocence, but the fangs of remorse tore into my bosom, and would not forego their hold" (Vol. I, chapter VII).

Here, Victor's conscience weighs on him so heavily that he believes that he feels even more tortured than Justine, because she is innocent and he, on some level, knows himself to be responsible for this crime.  

In the same chapter, he says,

"I, the true murderer, felt the never-dying worm alive in my bosom, which allowed of no hope or consolation [....].  Anguish and despair had penetrated into the core of my heart; I bore a hell within me, which nothing could extinguish."  

In other words, Victor feels that he is to blame for the deaths of both William and Justine, and there is nothing that can make him feel hopeful when he is so filled with sadness and sorrow. Again, these feelings seem to torture him as one would be tortured in hell. But instead of him being in hell, he feels that hell is inside him, that it is his own personal hell in which he suffers.

Later in the story, as Victor is relating the story of his travels with Clerval, he speaks to his now dead friend's soul, saying,

"your form so divinely wrought, and beaming with beauty, has decayed, but your spirit still visits and consoles your unhappy friend" (Vol. III, chapter I).  

Victor feels responsible for his friend's death as well, as it is the monster that strangles him, and he claims to be, still, quite unhappy as a result of all of these deaths.  They haunt him.