×The only time when you can kill adder with vertical/horizontal pointing sword is when it's 2 tiles.

Apart from that, you can also decrease adder's length by even number of tiles by trapping it with its body on one of hot tile squares.

2 is an even number, too, by the way.

×3 is an odd number. If adder's length is an odd number when you trap it on hot tile, then yes, you get and odd number as a result, because adder's length can be decreased only by an even number. Odd number minus even number is always odd number.

You, however, need an even number in the end.