Lightoller was there to the end, saw Murdoch got washed off. I'll doubt Murdoch killed himself in the water. Also, the claim of suicide happened before the ship sunk.
Sorry, Aly but that is not correct. Lightoller did not see Murdoch "washed off."
He thought he saw Murdoch working at collapsible A, just prior to Lightoller entering the water himself. Even that is in dispute - author George Behe noted when considering the evidence that "Lightoller was in no position to see Murdoch down on the starboard boat deck " (George Behe, First Officer Murdoch and the ‘Dalbeattie Defense ). Don Lynch also doubts his story - “Certainly Lightoller’s testimony [regarding Murdoch] can be discounted –he may well have been attempting to protect the reputation of a fellow White Star officer, as well as that of his employers.” (Illustrated History, p.195) . Paul Quinn even goes a step further -" something drew Lightoller over to the starboard side… did Murdoch lift the gun to his head and fire ” (Paul Quinn, Titanic at 2 A.M., p.77).
But based on what Lightoller did tell us - he did not see how Murdoch died. Anything else is simply assumption and speculation.
The reality, whether we like it or not, is that an officer most likely shot passengers/himself. We do not know for sure the identity of the officer, and it is impossible with the available evidence to rule Murdoch out of the equation.