#Shell + sed, 16 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i s/^/#/ $0 

Shell + sed, 16 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i s/^/#/ $0 

#Bash + sed, 16 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i s/^/#/ $0 

#Shell + sed, 16 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i s/^/#/ $0 

#Bash + sed, 19 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i 's/./#&/' $0 

#Bash + sed, 16 bytes

Not quite as destructive as some of the other answers ;-)

This script inserts a comment # at the beginning of every line of itself:

sed -i s/^/#/ $0