I've always enjoyed the idea of using a BTL as a murder weapon. Let's say a team gets a wetwork assignment, but the job can't actually look like a murder. The players would have to find out if the mark has a hot-sim enabled commlink, and if he doesn't they'd have to swap his commlink for one that does or modify his existing one. They would then replace the most commonly used sim-enabled program with a trip or mood snuff BTL with the intensity turned up to 11. Then, profit!
This becomes even easier in 4th and 5th edition where BTLs are available as direct download, which could presumably just be copied using the Edit File action. With a mark on the persona of the target, you might even be able to instruct one of his other devices to copy the file for you, making it look like he copied the file himself for all intents and purposes since he is now the owner of the file.
Here's a mechanical question related to direct downloads. The SR5 rulebook states that the normal simsense chips and the direct input chips both come with a security feature that erases the chip after a single use, and details how this can be disabled. Is it safe to assume that the download has a similar feature, and if so, can this be disabled in a similar way? Personally, I'd allow the same test as for chips, but using Software instead of Hardware. This seems like a fair compromise to me, given that it actually makes the Software skill more useful
