While I'm more knowledgeable about the day-to-day lives of Nicholas II and his family, I can give you some insight until someone else can step in.

You're right that he wasn't in St. Petersburg at the time, he was in Tsarskoe Selo which is about 15 miles away. As a result of this he wasn't as clued up on the situation in St. Petersburg as he might have been.

On Saturday, Father Gapon wrote a letter to Nicholas along with another addressed to the Minister of Internal Affairs, Svyatopolk-Mirsky. We know that the letter to Nicholas was delivered to the Winter Palace, but he'd left two days prior and so whether or not he was able to read it is unknown.

That evening, there was a cabinet meeting at the Winter Palace which Nicholas did not attend. They debated a few ideas about how to deal with the protestors, but all they really decided to do was place soldiers around the city centre in order to scare off the mob. Those in attendance of the meeting later wrote that the atmosphere had been quite calm and relaxed, and they'd all been fairly certain that just the presence of the soldiers would be enough to scare away the protestors.

After the meeting, Svyatopolk-Mirsky went to Tsarskoe Selo and told Nicholas about what had been discussed at the meeting. Again, everyone was calm and unconcerned.

Then, of course, the massacre happened. Nicholas wasn't told anything about it until Sunday evening, by which point it was much too late for him to intervene even if he wanted to.

So was he responsible? In my opinion, the answer is two-fold.

No, he didn't give the order for the soldiers to start firing. No, I don't believe he could have prevented it as of Sunday morning. Of that, at least, he is innocent.

But Nicholas famously hated the business of ruling. He saw his crown as a responsibility trusted to him by God—he hated being emperor, but he didn't want to delegate power because he felt certain in his heart that God had chosen him alone to rule Russia. He hated meetings with his ministers, he hated having to spend hours every day listening to reports. But I think it's a reasonable argument to say that had he been an even slightly more involved ruler, he would have seen the potential dangers and asked to be kept informed. If that had happened, he would have known the protestors were marching on the Winter Palace and could have stopped the shooting before it even began. The problem was that he was happy to let his ministers decide what he needed to know for him.

Nicholas bears responsibility for failing to prevent the massacre not in the moment itself, but in the years that led up to it. If he had stepped down and become the constitutional monarch of a democracy at any point prior to 1905, I don't believe Bloody Sunday would have happened. He might also have died an old man with grandchildren, but he just couldn't let go of his power.