You could hardly think of Dr Quimper as guilty, because every one of the Crackenthorpe family had a valid reason to kill.

valid 是形容词，来自拉丁词根 val-（表示"力量、价值"），加上后缀 -id 构成。基本意思是"有效的、有法律效力的"，引申为"合理的、正当的"。 在句中指"有充分依据的、站得住脚的"杀人理由。