OK, I'll bite on this one. Sorry, but I can be annoyingly logical.
When a sane adult makes a decision to do something wrong, they have typically done some mental evaluation of the risk vs reward of doing it and have decided that the reward outweighs the risk.
In this case, we know what the risk is. Evidence tampering is a felony in most states, I assume it is in NC as well. The maximum sentence that I see for this crime in many states is 10 years. I would guess that if caught, the person doing the tampering would also face perjury and possibly obstruction of justice charges. If multiple people are involved, there would also be conspiracy charges.
Thus, they would not only lose their job and their family's livelihood, they are facing prison time. Obviously they would never work in LE again and will face a challenge finding any employment after their sentence is complete as many companies do not hire convicted felons.
So, we understand the risk. What exactly is the reward that would outweigh this risk? Improving the odds of one single conviction in a career of investigations, arrests, and prosecutions? This isn't taking down the head of some major crime family or something similar that would be a "career making" case. Furthermore, it's not even guaranteeing a conviction. It's just boosting your chances.
What is the logical explanation of why a LEO would put everything he and his family have at risk to improve the chances of convicting BC?