That quote comes from his speech against Prussia going to war with the then weakened Austria. It had nothing to do with the "evil of war", he said it because Prussia's army wasn't anywhere near as strong as his colleagues thought it was at the time. The only people arguing for caution were the king and his court, he saw an opportunity.

He knew that arguing Prussia wasn't strong enough to defeat Austria would only enrage his peers' senses of national pride, probably committing them further to the idea of attacking.

He knew he was well known for his willingness to use military force and also known for his intelligence, so he gave this speech denouncing it and the power of that gesture convinced many politicians to change their votes. As a result, Prussia didn't go to war with Austria.

In the next few years the King came to rely on Bismark for many policy decisions and eventually made him prime minister. Under his leadership they bolstered the Prussian army... and invaded Austria, capturing it.

He was a genius, but also a calculating politician.