【问】 It was too late though，and I felt that the kindest thing I could do would be walk away.

(复旦大学主编《大学英语》，第1册第7课)

would be walk away是什么意思呢?

【答】 在动词walk之前省略了动词不定式符号to。动词不定式walk away在句子里作表语。更详细的情形，见to¹(动词不定式符号)。