It is not, as you'd suspect, hubris. Frankenstein is not "playing God": he should not be condemned for attempting to understand the source of life, nor for attempting to create life.
His tragic flaw is cowardice: he is unable or unwilling to confront the consequences of his science. His sin is not in creating the monster but in labeling it a monster; his sin is in abandoning and rejecting his creation the moment it frightened him.
And this is the theme that human beings must carry with us through all of our scientific endeavors.