For the Malaysians, especially their Prime Minister, they are defending their country against armed militants who want nothing more than occupy a land which they claim as theirs. For those who went there and put their lives at risk, they are just asserting a claim which they perceive as just because of their deep appreciation of History. Both sides have their own reasons why they must kill each other.
For the Malaysians, it is in defense of the Motherland. For the Tausug warriors who went there, it is honor and glory. The Malaysians have a very solid concept of the Motherland---the land with which they were born, those lands stringed by a federation borne from the collective will.
Study Malaysian history and you'll find that it is a place built by expatriates from other countries. Some history books claim that Malays pre-existed Filipinos while modern History shows otherwise--Malaysia's people came from the groups of homo sapiens who occupied the Philippine archiepelago first, before it occupied the Malay peninsula. Malaysia got its strength from a tenuous unity of races--Chinese, Indians, and Malays, such unity being treasured by everyone under a capitalist system which rewards the hard worker and punishes the recalcitrant lazy bone.
Malaysia's hard stance against the followers of the Sultan is consistent with its strong political will to keep this federation together. Sabah is part of its political subdivision and the Malaysians will throw hell's brimstones against those who threaten the unity of this police state.
Yes, the Tausugs have their rights under the Shariah and property rights are most prioritized. However, it would be next to impossible to annex Sabah back to Tausug or the Sultanate's possessions because, doing so, means destroying the very unity of the Malaysian federation. An armed occupation, honestly, is the last yet the most reasonable solution. Doing so, however, entails great risk of life and limb.
These kinds of issues are best solved through the negotiating table. The Sultanate must recognize the fact that it is next to impossible for them to annex Sabah, not until, the very peoples of Sabah want that.