This is a common source of loss for custodial services. Internal employees often have access to databases and servers hosting the company’s keys. The Shapeshift exchange was the victim of one such attack. Other exchanges have been attacked, not by employees, but by attackers that gain access to employee credentials or laptops. Employees are also in the best position to push out malicious software to their unwitting customers.
Not your keys, not your coins. Use sovereign products that give you key control.
Redundancy: your key management system should not depend on trusting a single manufacturer or software. For example, if using hardware wallets, the wallet firmware can provide checks and balances on the provider of your key management software