Will a Muslim who is killed wrongfully or treacherously or as a result of his house collapsing on him be a martyr?
Question

I know if a Muslim is killed by defending himself against an enemy then insha Allah he dies as a martyr. Also if he dies by drowning, stomach disease he also dies as a martyr. However if a Muslim is killed unexpectedly without being able to defend himself, for instance if he was killed from behind him would he still be considered as a martyr? Also in the case of the Gaza bombings, the Muslims that are killed by being bombed in their houses without expecting it, therefore not being able to defend themselves are they also considered as martyrs?.
Praise be to Allah.Firstly: 
Every Muslim who is killed wrongfully will have the reward of the martyr in the Hereafter, but in terms of this world, he is to be washed and the funeral prayer offered for him, and he is not to be dealt with in the same manner as one who was killed in battle. 
It says in al-Mawsoo’ah al-Fiqhiyyah (29/174):
The fuqaha’ are of the view that wrongdoing or injustice mean that the one who is killed in such ways is deemed a martyr, but not in the same sense as the martyr who is killed in battle with the kuffaar. Kinds of unlawful killing include one who is killed by thieves, criminals or bandits; one who is killed defending himself or his wealth or his life or his religion or his family or the Muslims or ahl al-dhimmah (non-Muslims living under Muslim rule); or one who is killed trying to prevent injustice; or one who dies in prison who has been imprisoned unlawfully. 
They differed as to whether he is regarded as a martyr in this world and in the Hereafter, or as a martyr in the Hereafter only. 
The majority of fuqaha’ were of the view that the one who is killed unlawfully is to be regarded as a martyr in the Hereafter only. He comes under the same ruling as the martyr in battle with the kuffaar in the Hereafter with regard to reward, but he does not come under the same ruling in this world, so he is to be washed (ghusl, after death) and the funeral prayer is to be offered for him. End quote. 
In order to attain the reward of the martyrs it is not essential for the one who is killed to have been in a confrontation with those aggressors. If they kill him unawares, then he will be deserving of the reward of the martyrs, in sha Allah. 
Among the evidence for that is the fact that ‘Umar ibn al-Khattaab was stabbed by Abu Lu’lu’ah al-Majoosi when he was leading the Muslims in Fajr prayer, and ‘Uthmaan ibn ‘Affaan was killed by those who had rebelled against him unlawfully, and the Prophet (blessings and peace of Allah be upon him) described them as martyrs. 
It was narrated that Anas ibn Maalik (may Allah be pleased with him) said: The Prophet (blessings and peace of Allah be upon him) climbed up Uhud, accompanied by Abu Bakr, ‘Umar and ‘Uthmaan, and the mountain shook with them. He struck it with his foot and said: “Stand firm, O Uhud, for there is no one on you but a Prophet or a Siddeeq or two martyrs.” Narrated by al-Bukhari (3483). 
Shaykh Muhammad ibn Saalih al-‘Uthaymeen (may Allah have mercy on him) said: By “Prophet” he was referring to himself, “Siddeeq” was Abu Bakr and the “two martyrs” were ‘Umar and ‘Uthmaan, both of whom (may Allah be pleased with them) were slain as martyrs. ‘Umar was killed when he was leading the Muslims in Fajr prayer, and he was killed in the mihraab. ‘Uthmaan was killed in his house. May Allah be pleased with them both and join us and the righteous Muslims with them in the abode of eternal bliss. End quote. 
Sharh Riyadh al-Saaliheen, 4/129, 130 
Secondly: 
With regard to our brothers in Gaza whose houses collapsed on them, we hope that they will be martyrs for two reasons: 
1.They were slain unlawfully
2.The Prophet (blessings and peace of Allah be upon him) said: “the one who is crushed beneath a falling wall is a martyr.” Narrated by al-Bukhari (2674) and Muslim (1914).
We would not be exaggerating if we add a third reason, which is that they were killed by the kuffaar on the battlefield, so they will be martyrs in this world and in the Hereafter, because all of Gaza was a battlefield. 
Prof. ‘Abd al-Rahmaan ibn Gharmaan ibn ‘Abd-Allah (may Allah preserve him) said: 
The view of the majority of Hanafis and Hanbalis, the correct view according to the Maalikis and the view of the Shaafa’is is that the one who is slain in war but not on the battlefield is a martyr in the general sense, regardless of the way in which he was killed, whether he was unaware or sleeping, whether he was engaged in fighting or not.  
What seems to me to be the case -- and Allah knows best -- is that the view of the majority is correct, because there is no clear evidence that the killing should be in battle. End quote. 
Ahkaam al-Shaheed fi’l-Fiqh al-Islami, 103-106 
We ask Allah to accept them as martyrs and to defeat the Jewish aggressors and enable us to prevail over them so that we might rule over them in accordance with His justice.  
And Allah knows best.