How a tough ATT would have helped

If a tough legally binding Arms Trade Treaty (ATT) based on all relevant international law, including human rights law, had been agreed by all UN Member States then this would have created stronger obligations on the exporter and transiting states, and would have been enforceable.
 
Obligations on original exporter

China would have been obliged to assess the transfer according to whether the arms were likely to be used to commit serious violations of human rights law or were likely to undermine sustainable development.

This assessment would be objective, making use of widely reported human rights abuses and instability in the global media. The human rights situation has also been reported by Amnesty International, Human Rights Watch, Zimbabwe Lawyers for Human Rights and Zimbabwe Association of Doctors for Human rights.

The original assessment would have determined that the arms should not be transferred to Zimbabwe at this time.

Obligations on transiting states

Even if the exporter disregarded the treaty, transit states would be required to assess the transfer through any part of their territory. Since Zimbabwe is landlocked, arms must pass through another country, either by land or by air. So a transit permit would be required before arms could be delivered, and the authorisation process would be the same as for the original export. If the arms were likely to be used to commit serious violations of human rights, or undermine sustainable development, the transit permit should be denied.

Common agreement on tough standards for arms transfers would also make it less likely that should one state refuse to transit the weapons, another state would do so instead.

Enforcement

As with any treaty, a tough ATT would have provided a legal framework to guide a series of legal and political measures ranging from diplomatic pressure to legal action potentially right up to the International Court of Justice. A mechanism for the settlement of disputes, as well as effective monitoring and enforcement mechanisms should be built into the treaty.