To what did Mr. Underwood liken Tom's death?

Mr. Underwood liken Tom's death to the senseless slaughter of songbirds.

Mr. Underwood simply figured it was a sin to kill cripples, be they standing, sitting, or escaping. He likened Tom’s death to the senseless slaughter of songbirds by hunters and children, and Maycomb thought he was trying to write an editorial poetical enough to be reprinted in The Montgomery Advertiser.