Why did scientists Albert Einstein and J Robert oppose development of a hydrogen bomb?

Answer 1

He thought it was immoral to use it. Quotes from Einstein about the bomb:

"We must not condemn man because his inventiveness and patient conquest of the forces of nature are exploited for false and destructive purposes.

"The line of demarcation doesn't lie between scientists and non-scientists; it lies between responsible, honest people, and the others.

"In our time, scientists and engineers carry a particularly heavy burden of moral responsibility, because the development of military means of mass destruction is dependent on their work.

"When men are engaged in war and conquest, the tools of science become as dangerous as a razor in the hands of a child."

Answer 2

They thought it would lead to an arms race.

Answer 3

opposed developing the H-bomb, claming it would only lead to perpetual arms race