Speed kills because of the far higher impact forces generated. To take this to its extreme: A car hitting another car at 10kmh is insignificant to a car hitting another car at 140kmh. Sure you can sight other extreme cases but like any form of transport, the faster you go the higher the impact forces.
Having ridden motorcycles for half my life I can tell speed causes accidents as well. The faster you go the smaller the margin of error you have to avoid a potential accident occurring. With Bikes they accelerate so hard that if you give it a fist full your's and other drivers margin of error drops dramatically.
