You are referring to Gerhard Schröder, who was the German chancellor from 1998 to 2005. So he was in charge of running the country, not the energy politics specifically in that time, and not in the 17 years following up today.

To start with, yes, his later (up to today) involvement with Gazprom is a scandal, bordering on corruption. But had nothing to do with German politics after 2005.

He might have been too friendly towards Russian gas, but Germany used Russian gas for many decades before him and the largest mistakes, which we suffer from today, have been made made by the governments which followed his. And I say this as someone who always opposed him.

While deciding about exiting nuclear energy long-term during his government, the switch to renewables was started. If the following governments had fully stuck to that, the situation would already be significantly different. And of course, however much gas you use, tying Germany so much to one supplier was the greatest mistake of all, especially after 2008 and 2014. There would have been ample time of adjusting energy politics to the changed situation, but nothing has been done.