Victor is an amazing scientist. Like many scientists (or at least like the stereotypical idea of the mad scientist), Victor becomes engulfed and obsessed with his goals...so much so that he loses the vision and contact with his loved ones. He even risks his life by allowing his health to falter.
Shelley is telling readers that science and advancements in this field are valuable, however, everything in moderation. To obsess about something to the point that the connection with nature (God) is lost, one's health is neglected, and contact with loved ones is broken is a sin. Science in this manner is dangerous because Victor played the role of God which no man can do without consequences. Victor learned this the hard way, and by telling his story to Robert Walton and also to us, the reader, he hopes that we will learn from the lesson without having to also experience the "sin" firsthand.
Victor Frankenstein becomes obsessed with the power that science provides him. As he begins to understand the components that make life possible, he insists upon trying to control them. He uses science to become God, and to create a creature of his own. That creature is a character of pity, a victim of his unorthodox creation, and because of his victimization, a monster determined to exact revenge. In other words, Victor set out to control nature and instead becomes a victim of it.
Shelley is telling her readers to be wary of science and the arrogance associated with knowledge. In a world becoming more and more technological, she is reminding her audience of the awesome power of nature, and cautioning them to be respectful of it.
It says that at the borders of our knowledge there is both brilliant triumph and disaster. Every time a scientists makes a discovery, part of society panics. Microwave Ovens were 'radioactive machines that would kill people'. The first trains were 'too fast and would pull your head off'. Cellphones give you brain cancer.
Last year, The Europeans started The Large Hadron Collidar, a massive physics experiment in Switzerland. The newspapers started claiming it will "destroy the entire planet if it goes wrong" and "the mad scientists are playing God with everyone's lives" and people panicked. But it was pure hysterical nonsense. There was no danger.
Beware of anyone who accuses scientists of 'playing God'. It is a meaningless phrase only designed to scare you. Science does have dangers and can be wrong. But we are all using their beautiful machines that have come from the work of geniuses struggling at the frontiers of knowledge (The internet, airplanes, cellphones, advanced healthcare, space exploration, etc etc.) In my experience, scientists are some of the most rational, responsible and careful members of society. They are also some of the most educated.
In the book Victor reanimates a dead body! The guy's a genius on the edge of knowledge using barely understood science and trying to kick start dead cells. The public says its 'playing God' and scientists are dangerous maniacs!?! But go to any hospital today and you'll see emergency medics using a machine called a defribrillator (see link). It does what Victor was trying to do. It uses electricity to reanimate people whose hearts have just died. It is Victor's experiment in action.
Are they are playing God or saving lives?