LED lights do not turn off completely? Why?
It's NOT dangerous, but annoying.  

It's floating/induced voltage that exists due to the travelers between the switches run in parallel. You can prove this by pulling just one travelers off a switch on either end of the switch circuit (or if standard toggle light switch putting one switch in the non-existent yet functional 'middle' position which breaks the connection and doesn't keep one traveler wire energized). 

One answer is buy higher quality bulbs, another is just ignore it.