Are grass strips more dangerous than tarmac?
Cloud, you are making a huge inference from a few small data points.  But in any case, to the basic question, which is actually a very good one:

In terms of the chance of getting killed, (what you seem to be looking for), maybe, marginally, but not enough to rule out operating from grass.  If you know what you are doing, and make allowances, I would say zero.  In terms of non-fatal incidents, maybe a bit higher, depending.   

On the plus side: A grass surface is more compliant than a paved one because the tires have less traction.  Grass is much easier to handle flying taildraggers because there is some lateral give with respect to tire side loading.  A tailwheel airplane on pavement is much more skittish and easier to ground loop.  This is also true of tri-gears but much less so because they are already naturally stable while rolling. In both cases however, most people will find it easier to maintain directional control while on grass because of the traction compliance.

On the down side: Somewhat less braking dry, quite a bit less braking wet (wet grass is quite slippery, as anyone who tries to run on it without spiked shoes quickly discovers).  Rolling resistance is higher so take-off distance can be longer than published numbers, which are based on a dry paved runway, and in marginal conditions this has to be accounted for and this varies with how long the grass is at the time.

Bottom line: If I'm in a taildragger I'll much prefer a grass runway. In a tricycle airplane I'll prefer a paved one.  Overall I would expect to see more runway excursion incidents on grass, like landings in rain or in morning dew where pilots slide off into a fence when they discover, surprise surprise, they can't stop, that kind of thing.

That being said, I would never say that a grass runway surface is inherently more dangerous IF the pilot is properly trained in what to expect (especially landing on wet) and makes allowances for it.  The only kind of fatality I would directly attribute to grass would be a takeoff fatality where the airplane's take-off run was extended and it struggled into the air at the far end and stalled out.

In the end though, regardless of this factor or that factor, these things really come down to that old standby, BAD AIRMANSHIP.  

More than boating, aviation is fraught with hidden hazards waiting to trip you up.  But if you are educated and trained, they are neutralized and can't get you unless you let your guard down.  You have to approach things in flying this way, or you will worry yourself out of the activity before you ever really get to enjoy it.  On the other hand, you are asking the right kind of questions so this is good.