On the one hand, the developers who were ultimately tasked directly with building Horizon were completely unqualified to write an accounting system, and lacked even basic knowledge about accounting in general, including fundamental misunderstandings about the very nature of double-entry and ledger-based accounting. From what I can remember from released correspondence, for example, Horizon had fundamental design mistakes that made it essentially not double-entry, particularly when multiple terminals were involved, even when not considering remote changes to accounts.

On the other, the severity of the consequences of the bugs in Horizon came from the behavior of Fujitsu management, the Post Office, and the judicial system, and I'm not sure that individual developers could have reasonably predicted that. The software was used under contracts that tried to make individual users personally liable even for shortfalls resulting from errors in the software. When accounts had shortfalls, the Post Office ignored even basic sanity in favor of insisting on Horizon's unerring accuracy. They abused esoteric powers of private investigation and private prosecution combined with their own vested interests to bring completely unreasonable prosecutions. They, along with parts of Fujitsu, repeatedly made false statements to courts about Horizon's basic operation, if often with enough distance from the actual developers to claim ignorance. The judicial system then operated under delusional and hubristic views on software development and practices around experts, witnesses, and coerced pleas that one might argue no reasonable person would have.

If a clearly negligent and unqualified engineer constructs part of an office building for a business with numerous avoidable tripping hazards that violate even basic standards, it seems reasonable that they might be liable for the injuries when employees trip on them. If it turns out that the business has a special right to shoot its employees with no consequences, decides that it would be better to shoot anyone who trips rather than admit to the building being fundamentally flawed, and then repeatedly has courts approve of its actions, I'm not so sure that the engineer should be held liable for mass murder.