My Questions for Great Expectations

"I held on tight, while Mrs. Joe and Joe ran to him. I didn’t know how I had done it, but I had no doubt I had murdered him somehow. In my dreadful situation, it was a relief when he was brought back, and, surveying the company all round as if they had disagreed with him, sank down into his chair with the one significant gasp, ‘Tar!’"(Ch. 4 pg. 47).

What I did not understand in this paragraph, was who did he 'murder'? Who is this, what relation does he have to Pip, and why did he say, 'Tar'? I really appreciate the help!

The Importance of Being Earnest

Here is the link for my entire paragraph:

https://docs.google.com/document/d/1ZS25oLh9Pso6BSJDbixlx9zfauN56PLSsXZsVrqFgfM/edit?hl=en#
 We definitely need to send another email so we can have all of them together and proofread them as one.