Is there a reason why the U.S. is trying to undermine the WTO and is trying to bypass the WTO to solve its trade grievances with China and other countries? What are the rationales and moral justifications for doing this?
Donald Trump believes that multilateral organizations block better bilateral agreements. So from his perspective, he can negotiate a better deal with China directly than through the World Trade Organization. History has shown that the US is more likely to lose a trade dispute in the WTO than win one. See What's Trump's frustration with the WTO?
Beyond all that, the WTO does not help unless there is an agreement between the countries for it to enforce. Absent such an agreement in the disputed area, the WTO has no ability to create agreements. From Understanding the WTO:
However, the point is not to pass judgement. The priority is to settle disputes, through consultations if possible. By January 2008, only about 136 of the nearly 369 cases had reached the full panel process. Most of the rest have either been notified as settled “out of court” or remain in a prolonged consultation phase — some since 1995.
So negotiating directly is the WTO process. In fact, the first step of any dispute is for the two parties to negotiate directly. And if the US (or any complainant) "wins" a dispute, it is up to the US to provide an enforcement mechanism. Basically the US would ask WTO permission to impose sanctions.
Donald Trump is not a fan of WTO and has previously threatened to leave the organization. Trying to resolve a major dispute with China through the WTO would go against his publicly proclaimed policies.