This same question was asked about the Apollo missions and while there were setbacks, none of the lunar missions experienced any fatalities.
Of course space travel is dangerous, and we have learned much since the Apollo days (1969-1972).
For instance, we know that extended time in a weightless environment is detrimental to the body's muscles and bones. Therefore, astronauts on the International Space Station (as well as Shuttle Missions) include exercise as part of their mission requirement.
We know that radiation is an even bigger concern than earlier believed. If a manned mission does venture to Mars someday, the crew of that craft will be out of the protective envelope of Earth's magnetic field (which helps divert intergalactic particles away from our world) much longer than the short lunar missions were. We're talking two years or more with a bombardment of much more hostile radiation. So this is certainly a challenge that needs to be addressed.
Another concern is psychological. The crew will not have much to do during the long interplanetary portion of the trip, which will last at least a year. They must have constructive duties to perform along with periodic mission-related drills to keep their training and skills sharp. They'll need to stay in touch with their families and friends too.
There will be a communication lag much greater than experienced with moon missions. It only took a little over a second for Apollo crews on the moon to get instructions from Houston.
With Mars perhaps 100 million miles away, we're talking eight or nine minutes for a message to make the trip, after which an answer must be sent taking equally long to return.
Therefore, in a real emergency the Mars crew would have to handle the initial distress without any advice from Earth for at least 16-18 minutes.
The Martian surface is hostile to life with extreme temperatures (not as bad as the Moon), and of course many unknowns.
While it's very unlikely there's life on the planet, there could be natural hazards as yet undetected. The thinner Martian atmosphere and the planet's closer proximity to the Asteroid Belt suggest a greater likelihood of occasional big rocks crashing in.
The crew will need a lot of oxygen, food and water to complete such a journey. A fuel cell will probably be used as it creates water and oxygen as part of its operation. A nuclear reactor (small one) may also be needed as solar energy is much weaker on Mars (although still usable), and energy requirements during the trip and once there will be great. This would pose a further radiation risk of course.
An accident such as happened with Apollo 13 would likely doom all of the astronauts to a lonely death in space many millions of miles from any help.
So yes! There are plenty of dangers in going to Mars. However, many test pilots have died through the years and yet aviation technology moved forward. Many medical volunteers did not get cured and yet medicine has made great advances. Danger is part of the price of progress.
If a crew is sent to Mars we owe it to them to provide them with the best safety technology offers, but we must also be candid: it is a gamble. Is it too dangerous? Absolutely not.
Such is the nature of exploration, which we can trace back all the way back to the Ancients.