It was rather thoughtless of him, to say the least.

至少可以说, 他太了。

It was thoughtless of you to eat all the cake and leave none for me.

你把蛋糕都吃了, 一点也不留给我, 这太自私了。

I wonder why you are so thoughtless of others.

我很奇怪, 你为何如此不替别人着想。