True, the personal liberty of the dead terrorist is very threatened.
No. Yours are. The terrorists are dead.
Apple sought to make their phones very secure for their customers so they did what no one else has done before with commercial encryption... they removed themselves from the equation. They don't have a key to the encryption. This is a personal device and even the manufacturer doesn't have the ability to pry into your private affairs.
The government doesn't like that and has been trying to change that through many varied attempts. This is another attempt. There is nothing likely to be gained from the iPhone that hasn't already been gained through cell carriers and ISPs... unless there are some incriminating duck-face selfies. This case gives them a wedge issue to get people to see Apple as the bad guy rather than the privacy advocates they are.
If the government gets their way and Apple devises a way to hack their own devices, then no one's device is secure anymore--from government or criminals.