Perhaps I don't understand what you mean by electrocutive, not being a native English speaker and all, but in my home country of the Netherlands over 90% of power lines have been underground since the 50s without any ill effects...
It's not a word, according to google ngram viewer. Two types of usage on google, one dealing with dance music and the other from 100+-year-old periodicals.
They're underground in many US cities, too, including San Francisco.
It creates a voltage gradient radiating out from the contact point. A gradient of more than 15VAC/m is generally bad news. The resistance can be enough to prevent automatic circuit protections from tripping as well if there's not a high current, such as dry high-resistance soil.
Downed power lines, yes.
Insulate them?
The cost of insulating a 765kV power line is prohibitively high.
Although in the modern day it might make sense to move more low voltage power lines underground than did previously.
Plus unless that insulation is made of something we haven't invented yet it's bound to flake and peel off in time.