Abu Hurayra related that, “The Messenger of Allah (blessings and peace be upon him) forbade making animals suffer.” [Bukhari]
The Prophet (Allah bless him & give him peace) said, ” Do not harm the creation of Allah.” [Abu Dawud, from al-Ma`rur ibn Suwayd. Imam Jamal al-Din al-Zayla`i declared it to be a rigorously authenticated (sahih) hadith in Nasb al-Raya.]
Ibn Hajar al-Haytami (Allah have mercy on him) mentions in his work on the major sins, al-Zawajir, that the scholars have explicitly stated that causing animals undue harm, such as hitting them painfully, without a sound reason is a major sin (kabira). [Ibn Hajar al-Haytami, al-Zawajir `an Iqtiraf al-Kaba'ir, 2.140]
So basically, don’t kill bugs unless it is necessary. What constitutes the necessity of killings bus? I have no idea. Personally, I have come to a point in my life where I don’t really care what the spider, fly, bee or any type of bug/insect is doing as long as it’s not crawling on me. If it’s crawling on me then, inna lillahi wa inna ilayhi rajioon (from Allah we came and unto Him we shall return).