Did the US Need to Fight in WW2?

Short answer: No.

I know that WW2 has been portrayed as a triumph of good over evil, and to some extent that is true. The Nazis and Imperial Japanese committed many atrocities and it is good thing that they were stopped. However, neither of them was a threat to the US and neither would have attacked the US had it remained neutral. 

Let's look at the neutrality part first. The US put various restrictions on Japan in response to Japan's attack on China in 1937: there was an embargo of oil & scrap metal, the Panama canal was closed to Japan, and trade links were cut. Later, the US began sending military aid to both Britain and China under the Lend Lease Act. This was a violation of international law of the time. The Hague Convention of 1907, which the US agreed to, gives laws for neutral nations during war. Article 6 of that treaty states:

"The supply, in any manner, directly or indirectly, by a neutral Power to a belligerent Power, of war-ships, ammunition, or war material of any kind whatever, is forbidden."

So the US failed to be neutral before the war and thus forfeited the protection given to neutral countries.

After Pearl Harbor, Japan and Germany declared war on the US and vice versa. German submarines sank many US merchant ships, but there was no danger of Germany ever invading the US. The Germans couldn't even get across the English channel. Even if the Germans had been able to invade, that chance was lost as soon as they invaded the USSR. So the US was in no danger of being conquered by Germany.

Japan had a similar problem. Even if they had been able to invade the US, the army was scattered around the Pacific and the bulk of it was tied up in China. So they could not have invaded either. The Japanese did not want to invade anyway- they were only interested in creating an empire in Asia. Their navy was a threat, but one which was easily defeated. The US broke their codes and destroyed most of their carriers at the Battle of Midway. The Japanese navy ceased to be a threat after that and spent the rest of the war losing every naval battle.

So, neither Germany nor Japan was a threat to the US and neither would have declared war on the US if it had stayed neutral. It's possible Germany could have defeated the USSR if the US had stayed neutral, but I doubt it. It's also possible Japan could have successfully built an empire in Asia, but again, I doubt it. No nation in history has ever succeeded in controlling that much territory for any length of time. Empires always collapse. 

I think the main lesson of WW2 is that the best way to maintain peace is to avoid alliances, sanctions, and do not send military aid to anyone. Sweden and Switzerland did this during WW2 and did not suffer from the war.